Merge pull request #60 from m1sk9/feat/i18n

feat: Support i18n

19 files changed, 541 insertions, 52 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7ea8c21..92c102f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,12 +1,11 @@
 # LunaticChat Changelog
 
-## v1 (Minecraft 1.21.X)
-
-### v1.0.0
+## v0
 
-- Dokka currently references only the platform-paper module.
+### v0.5.0
 
-## v0
+- Added i18n support for English and Japanese languages.
+- Fixed a Dokka currently references only the platform-paper module.
 
 ### v0.4.1
 
diff --git a/build.gradle.kts b/build.gradle.kts
index 31b2f87..26570b3 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -14,7 +14,7 @@ plugins {
 
 allprojects {
     group = "dev.m1sk9"
-    version = "1.0.0"
+    version = "0.5.0"
 
     repositories {
         mavenCentral()
diff --git a/docs/src/guide/configuration.md b/docs/src/guide/configuration.md
index 07458b3..3b7bd1e 100644
--- a/docs/src/guide/configuration.md
+++ b/docs/src/guide/configuration.md
@@ -24,6 +24,9 @@ userSettingsFilePath: "player-settings.yaml"
 # If enabled, LunaticChat will check for updates on startup.
 checkForUpdates: true
 
+# Plugin Configuration Language. This setting applies only to player feedback and does not affect plugin logs or similar outputs.
+language: "en"
+
 # ----------------------------------------------
 # -----------   Features Settings   ------------
 # ----------------------------------------------
@@ -88,6 +91,18 @@ LunaticChat がプレイヤーの設定を保存する YAML ファイルのパ�
 
 LunaticChat の起動時・権限を持ったプレイヤーがサーバに参加した際に，LunaticChat のアップデートを促すかどうか設定します．
 
+### `language`
+
+- Type: `string`
+- Default: `en`
+
+LunaticChat のプレイヤー向けメッセージの言語を指定します．
+
+### Supported languages:
+
+- `en`: English
+- `ja`: 日本語
+
 ## Features Settings
 
 ### `features.quickReplies.enabled`
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/LunaticChat.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/LunaticChat.kt
index 7a7365e..9664340 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/LunaticChat.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/LunaticChat.kt
@@ -14,6 +14,7 @@ import dev.m1sk9.lunaticChat.paper.config.ConfigManager
 import dev.m1sk9.lunaticChat.paper.config.LunaticChatConfiguration
 import dev.m1sk9.lunaticChat.paper.converter.ConversionCache
 import dev.m1sk9.lunaticChat.paper.converter.RomanjiConverter
+import dev.m1sk9.lunaticChat.paper.i18n.LanguageManager
 import dev.m1sk9.lunaticChat.paper.listener.PlayerChatListener
 import dev.m1sk9.lunaticChat.paper.listener.PlayerPresenceListener
 import dev.m1sk9.lunaticChat.paper.settings.PlayerSettingsManager
@@ -30,6 +31,7 @@ class LunaticChat :
     JavaPlugin(),
     Listener {
     lateinit var directMessageHandler: DirectMessageHandler
+    lateinit var languageManager: LanguageManager
 
     private lateinit var commandRegistry: CommandRegistry
     private var updateChecker: UpdateChecker? = null
@@ -47,6 +49,16 @@ class LunaticChat :
             logger.info("Debug: $configuration")
         }
 
+        // Initialize language manager (BEFORE commands)
+        languageManager =
+            LanguageManager(
+                plugin = this,
+                logger = logger,
+                selectedLanguage = configuration.language,
+            )
+        languageManager.initialize()
+        logger.info("Language system initialized: ${configuration.language.code}")
+
         val httpClient = HttpClient(CIO)
 
         // Initialize player settings manager (always needed for DM notifications)
@@ -176,21 +188,21 @@ class LunaticChat :
         commandRegistry = CommandRegistry(this)
 
         commandRegistry.registerAll(
-            TellCommand(this, directMessageHandler),
-            DirectMessageNoticeToggleCommand(this, playerSettingsManager!!),
+            TellCommand(this, directMessageHandler, languageManager),
+            DirectMessageNoticeToggleCommand(this, playerSettingsManager!!, languageManager),
         )
 
         // Register /reply command if quick replies are enabled
         if (configuration.features.quickRepliesEnabled.enabled) {
             commandRegistry.registerAll(
-                ReplyCommand(this, directMessageHandler),
+                ReplyCommand(this, directMessageHandler, languageManager),
             )
         }
 
         // Register /jp command if Japanese conversion is enabled
         if (configuration.features.japaneseConversion.enabled) {
             commandRegistry.registerAll(
-                RomajiConvertToggleCommand(this, playerSettingsManager!!),
+                RomajiConvertToggleCommand(this, playerSettingsManager!!, languageManager),
             )
         }
 
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/core/LunaticCommand.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/core/LunaticCommand.kt
index 8bbd1bd..d8ca412 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/core/LunaticCommand.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/core/LunaticCommand.kt
@@ -6,6 +6,8 @@ import dev.m1sk9.lunaticChat.paper.LunaticChat
 import dev.m1sk9.lunaticChat.paper.command.annotation.Command
 import dev.m1sk9.lunaticChat.paper.command.annotation.Permission
 import dev.m1sk9.lunaticChat.paper.command.annotation.PlayerOnly
+import dev.m1sk9.lunaticChat.paper.i18n.MessageFormatter
+import dev.m1sk9.lunaticChat.paper.i18n.MessageKey
 import io.papermc.paper.command.brigadier.CommandSourceStack
 import net.kyori.adventure.text.Component
 import net.kyori.adventure.text.format.NamedTextColor
@@ -38,8 +40,8 @@ abstract class LunaticCommand(
     /** Command aliases */
     val aliases: List<String> get() = commandAnnotation.aliases.toList()
 
-    /** Command description for help text */
-    val description: String get() = commandAnnotation.description
+    /** Command description for help text - can be overridden for i18n */
+    open val description: String get() = commandAnnotation.description
 
     /** Required permission node, if any */
     val permission: String? get() = permissionAnnotation?.value?.objectInstance?.permissionNode
@@ -75,9 +77,9 @@ abstract class LunaticCommand(
     protected fun checkPlayerOnly(ctx: CommandContext): CommandResult? {
         if (isPlayerOnly && !ctx.isPlayer) {
             return CommandResult.Failure(
-                Component
-                    .text("This command can only be executed by a player.")
-                    .color(NamedTextColor.RED),
+                MessageFormatter.formatError(
+                    plugin.languageManager.getMessage(MessageKey.PlayerOnlyCommand),
+                ),
             )
         }
 
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/DirectMessageNoticeToggleCommand.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/DirectMessageNoticeToggleCommand.kt
index 7906ca9..e838ee0 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/DirectMessageNoticeToggleCommand.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/DirectMessageNoticeToggleCommand.kt
@@ -9,23 +9,28 @@ import dev.m1sk9.lunaticChat.paper.command.annotation.Permission
 import dev.m1sk9.lunaticChat.paper.command.annotation.PlayerOnly
 import dev.m1sk9.lunaticChat.paper.command.core.CommandContext
 import dev.m1sk9.lunaticChat.paper.command.core.LunaticCommand
+import dev.m1sk9.lunaticChat.paper.i18n.LanguageManager
+import dev.m1sk9.lunaticChat.paper.i18n.MessageFormatter
+import dev.m1sk9.lunaticChat.paper.i18n.MessageKey
 import dev.m1sk9.lunaticChat.paper.settings.PlayerSettingsManager
 import io.papermc.paper.command.brigadier.CommandSourceStack
 import io.papermc.paper.command.brigadier.Commands
-import net.kyori.adventure.text.Component
-import net.kyori.adventure.text.format.NamedTextColor
 
 @Command(
     name = "notice",
     aliases = ["dmnotice"],
-    description = "Toggle direct message notification",
+    description = "",
 )
 @Permission(LunaticChatPermissionNode.NoticeToggle::class)
 @PlayerOnly
 class DirectMessageNoticeToggleCommand(
     plugin: LunaticChat,
     private val settingsManager: PlayerSettingsManager,
+    private val languageManager: LanguageManager,
 ) : LunaticCommand(plugin) {
+    override val description: String
+        get() = languageManager.getMessage(MessageKey.CommandDescriptionNotice)
+
     override fun buildCommand(): LiteralArgumentBuilder<CommandSourceStack> =
         Commands
             .literal(name)
@@ -63,11 +68,11 @@ class DirectMessageNoticeToggleCommand(
         val updatedSettings = currentSettings.copy(directMessageNotificationEnabled = enable)
         settingsManager.updateSettings(updatedSettings)
 
-        val statusText = if (enable) "enabled" else "disabled"
+        val toggleText = languageManager.getToggleText(enable)
         val message =
-            Component
-                .text("Direct message notification has been $statusText.")
-                .color(NamedTextColor.GREEN)
+            MessageFormatter.formatSuccess(
+                languageManager.getMessage(MessageKey.DirectMessageNoticeToggle(toggleText)),
+            )
 
         player.sendMessage(message)
         return CommandResult.Success
@@ -77,11 +82,11 @@ class DirectMessageNoticeToggleCommand(
         val player = ctx.requirePlayer()
         val settings = settingsManager.getSettings(player.uniqueId)
 
-        val statusText = if (settings.directMessageNotificationEnabled) "enabled" else "disabled"
+        val toggleText = languageManager.getToggleText(settings.directMessageNotificationEnabled)
         val message =
-            Component
-                .text("Direct message notification is currently $statusText.")
-                .color(NamedTextColor.YELLOW)
+            MessageFormatter.format(
+                languageManager.getMessage(MessageKey.DirectMessageNoticeStatus(toggleText)),
+            )
 
         player.sendMessage(message)
         return CommandResult.Success
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/ReplyCommand.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/ReplyCommand.kt
index 3d2fa88..0108f37 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/ReplyCommand.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/ReplyCommand.kt
@@ -11,23 +11,28 @@ import dev.m1sk9.lunaticChat.paper.command.annotation.PlayerOnly
 import dev.m1sk9.lunaticChat.paper.command.core.CommandContext
 import dev.m1sk9.lunaticChat.paper.command.core.LunaticCommand
 import dev.m1sk9.lunaticChat.paper.command.handler.DirectMessageHandler
+import dev.m1sk9.lunaticChat.paper.i18n.LanguageManager
+import dev.m1sk9.lunaticChat.paper.i18n.MessageFormatter
+import dev.m1sk9.lunaticChat.paper.i18n.MessageKey
 import io.papermc.paper.command.brigadier.CommandSourceStack
 import io.papermc.paper.command.brigadier.Commands
 import kotlinx.coroutines.runBlocking
-import net.kyori.adventure.text.Component
-import net.kyori.adventure.text.format.NamedTextColor
 
 @Command(
     name = "reply",
     aliases = ["r"],
-    description = "Reply to the last person who messaged you",
+    description = "",
 )
 @Permission(LunaticChatPermissionNode.Reply::class)
 @PlayerOnly
 class ReplyCommand(
     plugin: LunaticChat,
     private val dmHandler: DirectMessageHandler,
+    private val languageManager: LanguageManager,
 ) : LunaticCommand(plugin) {
+    override val description: String
+        get() = languageManager.getMessage(MessageKey.CommandDescriptionReply)
+
     override fun buildCommand(): LiteralArgumentBuilder<CommandSourceStack> =
         Commands
             .literal(name)
@@ -53,9 +58,9 @@ class ReplyCommand(
         val target =
             dmHandler.getReplyTarget(sender)
                 ?: return CommandResult.Failure(
-                    Component
-                        .text("You have no one to reply to.")
-                        .color(NamedTextColor.RED),
+                    MessageFormatter.formatError(
+                        languageManager.getMessage(MessageKey.ReplyTargetNotFound),
+                    ),
                 )
 
         runBlocking {
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/RomajiConvertToggleCommand.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/RomajiConvertToggleCommand.kt
index 5cf0492..b25667d 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/RomajiConvertToggleCommand.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/RomajiConvertToggleCommand.kt
@@ -9,23 +9,28 @@ import dev.m1sk9.lunaticChat.paper.command.annotation.Permission
 import dev.m1sk9.lunaticChat.paper.command.annotation.PlayerOnly
 import dev.m1sk9.lunaticChat.paper.command.core.CommandContext
 import dev.m1sk9.lunaticChat.paper.command.core.LunaticCommand
+import dev.m1sk9.lunaticChat.paper.i18n.LanguageManager
+import dev.m1sk9.lunaticChat.paper.i18n.MessageFormatter
+import dev.m1sk9.lunaticChat.paper.i18n.MessageKey
 import dev.m1sk9.lunaticChat.paper.settings.PlayerSettingsManager
 import io.papermc.paper.command.brigadier.CommandSourceStack
 import io.papermc.paper.command.brigadier.Commands
-import net.kyori.adventure.text.Component
-import net.kyori.adventure.text.format.NamedTextColor
 
 @Command(
     name = "jp",
     aliases = [],
-    description = "Toggle romaji to Japanese conversion for your messages",
+    description = "",
 )
 @Permission(LunaticChatPermissionNode.JapaneseToggle::class)
 @PlayerOnly
 class RomajiConvertToggleCommand(
     plugin: LunaticChat,
     private val settingsManager: PlayerSettingsManager,
+    private val languageManager: LanguageManager,
 ) : LunaticCommand(plugin) {
+    override val description: String
+        get() = languageManager.getMessage(MessageKey.CommandDescriptionJp)
+
     override fun buildCommand(): LiteralArgumentBuilder<CommandSourceStack> =
         Commands
             .literal(name)
@@ -63,11 +68,11 @@ class RomajiConvertToggleCommand(
         val updatedSettings = currentSettings.copy(japaneseConversionEnabled = enable)
         settingsManager.updateSettings(updatedSettings)
 
-        val statusText = if (enable) "enabled" else "disabled"
+        val toggleText = languageManager.getToggleText(enable)
         val message =
-            Component
-                .text("Japanese conversion has been $statusText.")
-                .color(NamedTextColor.GREEN)
+            MessageFormatter.formatSuccess(
+                languageManager.getMessage(MessageKey.RomajiConversionToggle(toggleText)),
+            )
 
         player.sendMessage(message)
         return CommandResult.Success
@@ -77,11 +82,11 @@ class RomajiConvertToggleCommand(
         val player = ctx.requirePlayer()
         val settings = settingsManager.getSettings(player.uniqueId)
 
-        val statusText = if (settings.japaneseConversionEnabled) "enabled" else "disabled"
+        val toggleText = languageManager.getToggleText(settings.japaneseConversionEnabled)
         val message =
-            Component
-                .text("Japanese conversion is currently $statusText.")
-                .color(NamedTextColor.YELLOW)
+            MessageFormatter.format(
+                languageManager.getMessage(MessageKey.RomajiConversionStatus(toggleText)),
+            )
 
         player.sendMessage(message)
         return CommandResult.Success
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/TellCommand.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/TellCommand.kt
index 36617a3..d958b23 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/TellCommand.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/command/impl/TellCommand.kt
@@ -13,25 +13,30 @@ import dev.m1sk9.lunaticChat.paper.command.annotation.PlayerOnly
 import dev.m1sk9.lunaticChat.paper.command.core.CommandContext
 import dev.m1sk9.lunaticChat.paper.command.core.LunaticCommand
 import dev.m1sk9.lunaticChat.paper.command.handler.DirectMessageHandler
+import dev.m1sk9.lunaticChat.paper.i18n.LanguageManager
+import dev.m1sk9.lunaticChat.paper.i18n.MessageFormatter
+import dev.m1sk9.lunaticChat.paper.i18n.MessageKey
 import io.papermc.paper.command.brigadier.CommandSourceStack
 import io.papermc.paper.command.brigadier.Commands
 import kotlinx.coroutines.runBlocking
-import net.kyori.adventure.text.Component
-import net.kyori.adventure.text.format.NamedTextColor
 import org.bukkit.Bukkit
 import java.util.concurrent.CompletableFuture
 
 @Command(
     name = "tell",
     aliases = ["t", "msg", "m", "w", "whisper"],
-    description = "Send a private message to another player",
+    description = "",
 )
 @Permission(LunaticChatPermissionNode.Tell::class)
 @PlayerOnly
 class TellCommand(
     plugin: LunaticChat,
     private val directMessageHandler: DirectMessageHandler,
+    private val languageManager: LanguageManager,
 ) : LunaticCommand(plugin) {
+    override val description: String
+        get() = languageManager.getMessage(MessageKey.CommandDescriptionTell)
+
     override fun buildCommand(): LiteralArgumentBuilder<CommandSourceStack> =
         Commands
             .literal(name)
@@ -64,16 +69,16 @@ class TellCommand(
         val recipient =
             Bukkit.getPlayer(targetName)
                 ?: return CommandResult.Failure(
-                    Component
-                        .text("Player '$targetName' is not online.")
-                        .color(NamedTextColor.RED),
+                    MessageFormatter.formatError(
+                        languageManager.getMessage(MessageKey.TellTargetOffline(targetName)),
+                    ),
                 )
 
         if (recipient.uniqueId == sender.uniqueId) {
             return CommandResult.Failure(
-                Component
-                    .text("You cannot send a message to yourself.")
-                    .color(NamedTextColor.RED),
+                MessageFormatter.formatError(
+                    languageManager.getMessage(MessageKey.TellYourself),
+                ),
             )
         }
 
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/ConfigManager.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/ConfigManager.kt
index 5f3a1b0..7661089 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/ConfigManager.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/ConfigManager.kt
@@ -4,6 +4,7 @@ import dev.m1sk9.lunaticChat.paper.config.key.FeaturesConfig
 import dev.m1sk9.lunaticChat.paper.config.key.JapaneseConversionFeatureConfig
 import dev.m1sk9.lunaticChat.paper.config.key.MessageFormatConfig
 import dev.m1sk9.lunaticChat.paper.config.key.QuickRepliesFeatureConfig
+import dev.m1sk9.lunaticChat.paper.i18n.Language
 import org.bukkit.configuration.file.FileConfiguration
 
 object ConfigManager {
@@ -59,6 +60,10 @@ object ConfigManager {
                         "userSettingsFilePath",
                         "player-settings.yaml",
                     )!!,
+                language =
+                    Language.fromCode(
+                        configFile.getString("language", "en")!!,
+                    ),
             )
 
         lunaticChatConfiguration = loadedConfig
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/LunaticChatConfiguration.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/LunaticChatConfiguration.kt
index 0dea2ac..92bc157 100644
--- a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/LunaticChatConfiguration.kt
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/config/LunaticChatConfiguration.kt
@@ -2,6 +2,7 @@ package dev.m1sk9.lunaticChat.paper.config
 
 import dev.m1sk9.lunaticChat.paper.config.key.FeaturesConfig
 import dev.m1sk9.lunaticChat.paper.config.key.MessageFormatConfig
+import dev.m1sk9.lunaticChat.paper.i18n.Language
 
 data class LunaticChatConfiguration(
     val features: FeaturesConfig,
@@ -9,4 +10,5 @@ data class LunaticChatConfiguration(
     val debug: Boolean = false,
     val userSettingsFilePath: String = "player-settings.yaml",
     val checkForUpdates: Boolean = true,
+    val language: Language = Language.EN,
 )
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/Language.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/Language.kt
new file mode 100644
index 0000000..a25ecab
--- /dev/null
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/Language.kt
@@ -0,0 +1,27 @@
+package dev.m1sk9.lunaticChat.paper.i18n
+
+/**
+ * Represents a supported language in LunaticChat.
+ *
+ * @property code The language code (e.g., "en", "ja")
+ * @property fileName The name of the language file (e.g., "en.yml", "ja.yml")
+ */
+enum class Language(
+    val code: String,
+    val fileName: String,
+) {
+    EN("en", "en.yml"),
+    JA("ja", "ja.yml"),
+    ;
+
+    companion object {
+        /**
+         * Converts a language code string to a [Language] enum.
+         * If the code is not recognized, returns [EN] as a fallback.
+         *
+         * @param code The language code to convert
+         * @return The corresponding [Language] enum, or [EN] if not found
+         */
+        fun fromCode(code: String): Language = entries.find { it.code.equals(code, ignoreCase = true) } ?: EN
+    }
+}
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/LanguageConfig.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/LanguageConfig.kt
new file mode 100644
index 0000000..cc531ab
--- /dev/null
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/LanguageConfig.kt
@@ -0,0 +1,58 @@
+package dev.m1sk9.lunaticChat.paper.i18n
+
+import kotlinx.serialization.Serializable
+
+/**
+ * Represents the structure of a language file.
+ * This data class mirrors the YAML structure of language files (en.yml, ja.yml).
+ *
+ * @property commandDescription Command descriptions for various commands
+ * @property directMessageNoticeStatus Status message for direct message notifications
+ * @property directMessageNoticeToggle Toggle message for direct message notifications
+ * @property replyTargetNotFound Error message when reply target is not found
+ * @property romajiConversionStatus Status message for romaji conversion
+ * @property romajiConversionToggle Toggle message for romaji conversion
+ * @property tellTargetOffline Error message when tell target is offline
+ * @property tellYourself Error message when trying to message yourself
+ * @property toggle Toggle state messages (on/off)
+ */
+@Serializable
+data class LanguageConfig(
+    val commandDescription: CommandDescriptions,
+    val directMessageNoticeStatus: String,
+    val directMessageNoticeToggle: String,
+    val replyTargetNotFound: String,
+    val romajiConversionStatus: String,
+    val romajiConversionToggle: String,
+    val tellTargetOffline: String,
+    val tellYourself: String,
+    val toggle: ToggleMessages,
+)
+
+/**
+ * Command descriptions for all LunaticChat commands.
+ *
+ * @property jp Description for the /jp command
+ * @property notice Description for the /notice command
+ * @property reply Description for the /reply command
+ * @property tell Description for the /tell command
+ */
+@Serializable
+data class CommandDescriptions(
+    val jp: String,
+    val notice: String,
+    val reply: String,
+    val tell: String,
+)
+
+/**
+ * Toggle state messages.
+ *
+ * @property on Message for "enabled" state
+ * @property off Message for "disabled" state
+ */
+@Serializable
+data class ToggleMessages(
+    val on: String,
+    val off: String,
+)
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/LanguageManager.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/LanguageManager.kt
new file mode 100644
index 0000000..f355808
--- /dev/null
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/LanguageManager.kt
@@ -0,0 +1,104 @@
+package dev.m1sk9.lunaticChat.paper.i18n
+
+import com.charleskorn.kaml.Yaml
+import org.bukkit.plugin.java.JavaPlugin
+import java.util.logging.Logger
+
+/**
+ * Manages language files and provides message retrieval with type safety.
+ *
+ * This class handles:
+ * - Loading language files from resources/languages/
+ * - Caching language configurations in memory
+ * - Providing message retrieval with placeholder substitution
+ * - Fallback to English if the selected language is unavailable
+ *
+ * @property plugin The JavaPlugin instance
+ * @property logger The logger for this manager
+ * @property selectedLanguage The currently selected language
+ */
+class LanguageManager(
+    private val plugin: JavaPlugin,
+    private val logger: Logger,
+    private val selectedLanguage: Language,
+) {
+    private val languageCache = mutableMapOf<Language, LanguageConfig>()
+
+    /**
+     * Initializes the language manager by loading all language files.
+     * This should be called during plugin initialization.
+     *
+     * @throws IllegalStateException if the English fallback file is missing or cannot be loaded
+     */
+    fun initialize() {
+        Language.entries.forEach { lang ->
+            try {
+                val config = loadLanguageFile(lang)
+                languageCache[lang] = config
+                logger.info("Loaded language file: ${lang.fileName}")
+            } catch (e: Exception) {
+                logger.warning("Failed to load ${lang.fileName}: ${e.message}")
+                if (lang == Language.EN) {
+                    throw IllegalStateException("English fallback missing", e)
+                }
+            }
+        }
+    }
+
+    /**
+     * Loads a language file from resources/languages/.
+     *
+     * @param language The language to load
+     * @return The parsed LanguageConfig
+     * @throws IllegalStateException if the language file is not found
+     */
+    private fun loadLanguageFile(language: Language): LanguageConfig {
+        val stream =
+            plugin.getResource("languages/${language.fileName}")
+                ?: throw IllegalStateException("Language file not found: languages/${language.fileName}")
+
+        val yamlContent = stream.bufferedReader().use { it.readText() }
+        return Yaml.default.decodeFromString(LanguageConfig.serializer(), yamlContent)
+    }
+
+    /**
+     * Retrieves a message for the given message key.
+     * If the selected language is unavailable, falls back to English.
+     *
+     * @param key The message key to retrieve
+     * @return The formatted message with placeholders substituted
+     */
+    fun getMessage(key: MessageKey): String {
+        val config = languageCache[selectedLanguage] ?: languageCache[Language.EN]!!
+
+        return when (key) {
+            MessageKey.CommandDescriptionTell -> config.commandDescription.tell
+            MessageKey.CommandDescriptionReply -> config.commandDescription.reply
+            MessageKey.CommandDescriptionJp -> config.commandDescription.jp
+            MessageKey.CommandDescriptionNotice -> config.commandDescription.notice
+            is MessageKey.DirectMessageNoticeStatus ->
+                config.directMessageNoticeStatus.replace("{toggle}", key.toggle)
+            is MessageKey.DirectMessageNoticeToggle ->
+                config.directMessageNoticeToggle.replace("{toggle}", key.toggle)
+            is MessageKey.RomajiConversionStatus ->
+                config.romajiConversionStatus.replace("{toggle}", key.toggle)
+            is MessageKey.RomajiConversionToggle ->
+                config.romajiConversionToggle.replace("{toggle}", key.toggle)
+            MessageKey.ReplyTargetNotFound -> config.replyTargetNotFound
+            is MessageKey.TellTargetOffline ->
+                config.tellTargetOffline.replace("{target}", key.targetName)
+            MessageKey.TellYourself -> config.tellYourself
+            MessageKey.ToggleOn -> config.toggle.on
+            MessageKey.ToggleOff -> config.toggle.off
+            MessageKey.PlayerOnlyCommand -> "This command can only be executed by players."
+        }
+    }
+
+    /**
+     * Gets the translated text for a toggle state (enabled/disabled).
+     *
+     * @param enabled The toggle state
+     * @return The translated "on" or "off" text
+     */
+    fun getToggleText(enabled: Boolean): String = getMessage(if (enabled) MessageKey.ToggleOn else MessageKey.ToggleOff)
+}
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/MessageFormatter.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/MessageFormatter.kt
new file mode 100644
index 0000000..862ab61
--- /dev/null
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/MessageFormatter.kt
@@ -0,0 +1,129 @@
+package dev.m1sk9.lunaticChat.paper.i18n
+
+import net.kyori.adventure.text.Component
+import net.kyori.adventure.text.format.NamedTextColor
+
+/**
+ * Formats messages with the prefix and applies color styling.
+ *
+ * All messages follow this format:
+ * - Prefix: `LC`, AQUA + BOLD
+ * - Message: Main text in GRAY
+ * - Placeholders: Text in {braces} highlighted in YELLOW
+ */
+object MessageFormatter {
+    private val PREFIX_COLOR = NamedTextColor.LIGHT_PURPLE
+    private val MESSAGE_COLOR = NamedTextColor.WHITE
+    private val PLACEHOLDER_COLOR = NamedTextColor.YELLOW
+    private val ERROR_COLOR = NamedTextColor.RED
+    private val SUCCESS_COLOR = NamedTextColor.GREEN
+
+    /**
+     * Formats a message with the standard prefix and gray text.
+     * Placeholders in {braces} are highlighted in yellow if [highlightPlaceholders] is true.
+     *
+     * @param message The message text to format
+     * @param highlightPlaceholders Whether to highlight {placeholder} text in yellow
+     * @return A formatted Component with the prefix
+     */
+    fun format(
+        message: String,
+        highlightPlaceholders: Boolean = true,
+    ): Component {
+        val prefix =
+            Component
+                .text("[LC] ")
+                .color(PREFIX_COLOR)
+
+        val messageComponent =
+            if (highlightPlaceholders) {
+                formatWithPlaceholders(message, MESSAGE_COLOR)
+            } else {
+                Component.text(message).color(MESSAGE_COLOR)
+            }
+
+        return prefix.append(messageComponent)
+    }
+
+    /**
+     * Formats an error message with the prefix and red text.
+     *
+     * @param message The error message text
+     * @return A formatted Component with red text
+     */
+    fun formatError(message: String): Component {
+        val prefix =
+            Component
+                .text("[LC] ")
+                .color(PREFIX_COLOR)
+
+        val messageComponent = formatWithPlaceholders(message, ERROR_COLOR)
+
+        return prefix.append(messageComponent)
+    }
+
+    /**
+     * Formats a success message with the prefix and green text.
+     *
+     * @param message The success message text
+     * @return A formatted Component with green text
+     */
+    fun formatSuccess(message: String): Component {
+        val prefix =
+            Component
+                .text("[LC] ")
+                .color(PREFIX_COLOR)
+
+        val messageComponent = formatWithPlaceholders(message, SUCCESS_COLOR)
+
+        return prefix.append(messageComponent)
+    }
+
+    /**
+     * Parses a message and highlights placeholders in {braces} with yellow color.
+     * Text outside braces uses the specified base color.
+     *
+     * @param message The message text to parse
+     * @param baseColor The color for non-placeholder text
+     * @return A Component with highlighted placeholders
+     */
+    private fun formatWithPlaceholders(
+        message: String,
+        baseColor: NamedTextColor,
+    ): Component {
+        val result = Component.text()
+        val regex = Regex("""\{([^}]+)}""")
+        var lastIndex = 0
+
+        regex.findAll(message).forEach { match ->
+            // Add text before the placeholder
+            if (match.range.first > lastIndex) {
+                result.append(
+                    Component
+                        .text(message.substring(lastIndex, match.range.first))
+                        .color(baseColor),
+                )
+            }
+
+            // Add the placeholder in yellow (including braces)
+            result.append(
+                Component
+                    .text(match.value)
+                    .color(PLACEHOLDER_COLOR),
+            )
+
+            lastIndex = match.range.last + 1
+        }
+
+        // Add remaining text after the last placeholder
+        if (lastIndex < message.length) {
+            result.append(
+                Component
+                    .text(message.substring(lastIndex))
+                    .color(baseColor),
+            )
+        }
+
+        return result.build()
+    }
+}
diff --git a/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/MessageKey.kt b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/MessageKey.kt
new file mode 100644
index 0000000..b6fe8dc
--- /dev/null
+++ b/platform-paper/src/main/kotlin/dev/m1sk9/lunaticChat/paper/i18n/MessageKey.kt
@@ -0,0 +1,53 @@
+package dev.m1sk9.lunaticChat.paper.i18n
+
+/**
+ * Type-safe message keys for LunaticChat i18n system.
+ *
+ * This sealed class ensures compile-time safety when referencing messages,
+ * preventing typos and missing translations. Each message key can have
+ * associated parameters that will be substituted in the final message.
+ */
+sealed class MessageKey {
+    // Command descriptions
+    data object CommandDescriptionTell : MessageKey()
+
+    data object CommandDescriptionReply : MessageKey()
+
+    data object CommandDescriptionJp : MessageKey()
+
+    data object CommandDescriptionNotice : MessageKey()
+
+    // Toggle messages (with placeholder)
+    data class DirectMessageNoticeStatus(
+        val toggle: String,
+    ) : MessageKey()
+
+    data class DirectMessageNoticeToggle(
+        val toggle: String,
+    ) : MessageKey()
+
+    data class RomajiConversionStatus(
+        val toggle: String,
+    ) : MessageKey()
+
+    data class RomajiConversionToggle(
+        val toggle: String,
+    ) : MessageKey()
+
+    // Error messages
+    data object ReplyTargetNotFound : MessageKey()
+
+    data class TellTargetOffline(
+        val targetName: String,
+    ) : MessageKey()
+
+    data object TellYourself : MessageKey()
+
+    // Toggle values
+    data object ToggleOn : MessageKey()
+
+    data object ToggleOff : MessageKey()
+
+    // System messages
+    data object PlayerOnlyCommand : MessageKey()
+}
diff --git a/platform-paper/src/main/resources/config.yml b/platform-paper/src/main/resources/config.yml
index e6a372d..331f690 100644
--- a/platform-paper/src/main/resources/config.yml
+++ b/platform-paper/src/main/resources/config.yml
@@ -21,6 +21,9 @@ userSettingsFilePath: "player-settings.yaml"
 # If enabled, LunaticChat will check for updates on startup.
 checkForUpdates: true
 
+# Plugin Configuration Language. This setting applies only to player feedback and does not affect plugin logs or similar outputs.
+language: "en"
+
 # ----------------------------------------------
 # -----------   Features Settings   ------------
 # ----------------------------------------------
diff --git a/platform-paper/src/main/resources/languages/en.yml b/platform-paper/src/main/resources/languages/en.yml
new file mode 100644
index 0000000..17743f2
--- /dev/null
+++ b/platform-paper/src/main/resources/languages/en.yml
@@ -0,0 +1,30 @@
+# ----------------------------------------------
+# ------  LunaticChat Languages Files   --------
+# -----------------  English   -----------------
+# ----------------------------------------------
+#
+# English language file for LunaticChat i18n (internationalization)
+#
+# These translations may affect the UI/UX of LunaticChat, so please translate accurately.
+#
+# When adding new translation keys, please submit a pull request to GitHub.
+#
+# ----------------------------------------------
+
+commandDescription:
+  jp: "Toggle romaji-to-kana conversion on/off"
+  notice: "Toggle direct message notifications on/off"
+  reply: "Reply to the player who last sent/received a direct message"
+  tell: "Send a direct message to another player"
+
+directMessageNoticeStatus: "Direct message notifications are currently {toggle}"
+directMessageNoticeToggle: "Direct message notifications have been set to {toggle}"
+replyTargetNotFound: "Reply target player not found."
+romajiConversionStatus: "Romaji-to-kana conversion is currently {toggle}"
+romajiConversionToggle: "Romaji-to-kana conversion has been set to {toggle}"
+tellTargetOffline: "Player '{target}' is currently offline."
+tellYourself: "You cannot send a message to yourself."
+
+toggle:
+  off: "Disabled"
+  on: "Enabled"
diff --git a/platform-paper/src/main/resources/languages/ja.yml b/platform-paper/src/main/resources/languages/ja.yml
new file mode 100644
index 0000000..e8770f8
--- /dev/null
+++ b/platform-paper/src/main/resources/languages/ja.yml
@@ -0,0 +1,30 @@
+# ----------------------------------------------
+# ------  LunaticChat Languages Files   --------
+# -----------------  日本語   -------------------
+# ----------------------------------------------
+#
+# LunaticChat の i18n (国際化) 用の日本語言語ファイル
+#
+# 以下の翻訳は LunaticChat の UI/UX に影響を与える可能性があるため、正確に翻訳してください。
+#
+# また，新しい翻訳キーを追加する際は GitHub へのプルリクエストの提出をお願いします。
+#
+# ----------------------------------------------
+
+toggle:
+  on: "有効"
+  off: "無効"
+
+commandDescription:
+  tell: "他のプレイヤーにダイレクトメッセージを送信します"
+  reply: "最後にダイレクトメッセージを送信/受信したプレイヤーに返信します"
+  jp: "かな・ローマ字変換機能のオン/オフを切り替えます"
+  notice: "ダイレクトメッセージ通知のオン/オフを切り替えます"
+
+directMessageNoticeToggle: "ダイレクトメッセージ通知を{toggle}にしました"
+directMessageNoticeStatus: "現在ダイレクトメッセージ通知は{toggle}です"
+replyTargetNotFound: "返信対象のプレイヤーが見つかりません"
+romajiConversionToggle: "かな・ローマ字変換機能を{toggle}にしました"
+romajiConversionStatus: "現在かな・ローマ字変換機能は{toggle}です"
+tellTargetOffline: "プレイヤー '{target}' は現在オフラインです"
+tellYourself: "自分自身にメッセージを送信することはできません"