Localize messages using ResourceBundles

Messages can be localized by calling `L10n.getMessage`, which also
formats named arguments. Furthermore the extension function
`Player.getMessage` determines the locale to use by invoking
`L10n.localeProvider`.

src/main/kotlin/net/axay/kspigot/localization/KSpigotLocalization.kt

@@ -0,0 +1,81 @@
+package net.axay.kspigot.localization
+
+import org.apache.commons.lang.text.StrSubstitutor
+import org.bukkit.entity.Player
+import java.io.InputStreamReader
+import java.nio.charset.StandardCharsets
+import java.util.*
+import kotlin.collections.HashMap
+
+private const val PREFIX = "{"
+private const val SUFFIX = "}"
+
+/**
+ * Handles localization of strings using java [ResourceBundle]s.
+ *
+ * Message property files should reside in `src/main/resources` named
+ * `messages_locale.properties`, where `locale` is formatted as returned by
+ * [Locale.toString].
+ */
+object L10n {
+    /**
+     * This function determines which locale is used for a player, by default
+     * [Locale.US] is returned.
+     *
+     * It is invoked every time a message is localized, so some sort of caching
+     * is advisable in many cases.
+     */
+    var localeProvider: (Player) -> Locale = { Locale.US }
+
+    private val bundles: MutableMap<Locale, ResourceBundle> = HashMap()
+
+    /**
+     * Returns the localized string for [key]
+     *
+     * @throws MissingResourceException if no messages exist for the given
+     * locale or [key] was not found in the messages
+     */
+    fun getMessage(locale: Locale, key: String): String =
+        getOrLoadBundle(locale)?.getString(key) ?: throw MissingResourceException(
+            "Messages for locale '$locale' not found", "ResourceBundle", key
+        )
+
+    /**
+     * Additionally formats the localized string with the named [args], which
+     * are represented as `{argumentName}` in the translations
+     */
+    fun getMessage(locale: Locale, key: String, vararg args: Pair<String, Any?>): String =
+        StrSubstitutor.replace(
+            getMessage(locale, key),
+            mapOf(*args),
+            PREFIX, SUFFIX
+        )
+
+    private fun getOrLoadBundle(locale: Locale): ResourceBundle? {
+        return bundles[locale] ?: InputStreamReader(
+            javaClass
+                .classLoader
+                .getResourceAsStream("messages_$locale.properties") ?: return null,
+            StandardCharsets.UTF_8
+        ).use {
+            val bundle = PropertyResourceBundle(it)
+            bundles[locale] = bundle
+            bundle
+        }
+    }
+}
+
+/**
+ * Returns the localized message using the locale provided by
+ * [L10n.localeProvider]
+ *
+ * @see L10n.getMessage
+ */
+fun Player.getMessage(key: String) =
+    L10n.getMessage(L10n.localeProvider(this), key)
+
+/**
+ * @see L10n.getMessage
+ */
+fun Player.getMessage(key: String, vararg args: Pair<String, Any?>) =
+    L10n.getMessage(L10n.localeProvider(this), key, *args)