SPIGOT-439: Add player locale related APIs.

By: MiniDigger <admin@minidigger.me>

paper-api/src/main/java/org/bukkit/entity/Player.java

@@ -1403,4 +1403,17 @@ public interface Player extends HumanEntity, Conversable, CommandSender, Offline
      * @return object detailing the player's progress
      */
     public AdvancementProgress getAdvancementProgress(Advancement advancement);
+
+    /**
+     * Gets the player's current locale.
+     *
+     * The value of the locale String is not defined properly.
+     * <br>
+     * The vanilla Minecraft client will use lowercase language / country pairs
+     * separated by an underscore, but custom resource packs may use any format
+     * they wish.
+     *
+     * @return the player's locale
+     */
+    public String getLocale();
 }

paper-api/src/main/java/org/bukkit/event/player/PlayerLocaleChangeEvent.java

@@ -0,0 +1,37 @@
+package org.bukkit.event.player;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.HandlerList;
+
+/**
+ * Called when a player changes their locale in the client settings.
+ */
+public class PlayerLocaleChangeEvent extends PlayerEvent {
+
+    private static final HandlerList handlers = new HandlerList();
+    //
+    private final String locale;
+
+    public PlayerLocaleChangeEvent(Player who, String locale) {
+        super(who);
+        this.locale = locale;
+    }
+
+    /**
+     * @see Player#getLocale()
+     *
+     * @return the player's new locale
+     */
+    public String getLocale() {
+        return locale;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}