add /language command (#780)

* 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

src/main/java/emu/grasscutter/Grasscutter.java

@@ -206,6 +206,14 @@ public final class Grasscutter {
 		return language;
 	}
 
+	public static void setLanguage(Language language) {
+        Grasscutter.language = language;
+	}
+
+	public static Language getLanguage(String langCode) {
+        return Language.getLanguage(langCode);
+	}
+
 	public static Logger getLogger() {
 		return log;
 	}