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

src/main/java/emu/grasscutter/utils/Utils.java

@@ -9,6 +9,7 @@ import java.time.temporal.TemporalAdjusters;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Random;
+import java.util.Locale;
 
 import emu.grasscutter.Config;
 import emu.grasscutter.Grasscutter;
@@ -306,4 +307,12 @@ public final class Utils {
 
 		return map;
 	}
+
+	/**
+	 * get language code from Locale
+	 */
+    public static String getLanguageCode(Locale locale) {
+        return String.format("%s-%s", locale.getLanguage(), locale.getCountry());
+    }
+
 }