fixed pr780, uploaded LanguageCommand.java (#782)

* Fix the following issues:
1. HashMap non-thread-safe issus
2. Fix the same problem in pr621, but use a better implementation

Add the following functions:
1. There is now a language cache inside getLanguage to prepare for different languages corresponding to different time zones where the accounts in the server are located

* add /language command,each account has their own Locate

* I forgot to git add...sorry,,this pr is to fix pr780, uploaded LanguageCommand.java

src/main/java/emu/grasscutter/command/commands/LanguageCommand.java

+package emu.grasscutter.command.commands;
+
+import emu.grasscutter.Grasscutter;
+import emu.grasscutter.command.Command;
+import emu.grasscutter.command.CommandHandler;
+import emu.grasscutter.database.DatabaseHelper;
+import emu.grasscutter.game.Account;
+import emu.grasscutter.game.player.Player;
+import emu.grasscutter.utils.Utils;
+
+import java.util.List;
+import java.util.Locale;
+
+import static emu.grasscutter.utils.Language.translate;
+
+@Command(label = "language", usage = "language [language code]", description = "commands.language.description", aliases = {"lang"})
+public final class LanguageCommand implements CommandHandler {
+
+    @Override
+    public void execute(Player sender, Player targetPlayer, List<String> args) {
+        if (args.isEmpty()) {
+            String curLangCode = null;
+            if (sender != null) {
+                curLangCode = Utils.getLanguageCode(sender.getAccount().getLocale());
+            }
+            else {
+                curLangCode = Grasscutter.getLanguage().getLanguageCode();
+            }
+            CommandHandler.sendMessage(sender, translate(sender, "commands.language.current_language", curLangCode));
+            return;
+        }
+
+        String langCode = args.get(0);
+        String actualLangCode = null;
+        if (sender != null) {
+            var locale = Locale.forLanguageTag(langCode);
+            actualLangCode = Utils.getLanguageCode(locale);
+            sender.getAccount().setLocale(locale);
+            return;
+        }
+        else {
+            var languageInst = Grasscutter.getLanguage(langCode);
+            actualLangCode = languageInst.getLanguageCode();
+            Grasscutter.setLanguage(languageInst);
+        }
+        CommandHandler.sendMessage(sender, translate(sender, "commands.language.language_changed", actualLangCode));
+
+    }
+}