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/Config.java

@@ -22,7 +22,7 @@ public final class Config {
 	public GameServerOptions GameServer = new GameServerOptions();
 	public DispatchServerOptions DispatchServer = new DispatchServerOptions();
 	public Locale LocaleLanguage = Locale.getDefault();
-	public Locale DefaultLanguage = Locale.ENGLISH;
+	public Locale DefaultLanguage = Locale.US;
 
 	public Boolean OpenStamina = true;
 	public GameServerOptions getGameServerOptions() {

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

@@ -149,14 +149,7 @@ public final class Grasscutter {
 
 	public static void loadLanguage() {
 		var locale = config.LocaleLanguage;
-		var languageTag = locale.toLanguageTag();
-		
-		if (languageTag.equals("und")) {
-			Grasscutter.getLogger().error("Illegal locale language, using 'en-US' instead.");
-			language = Language.getLanguage("en-US");
-		} else {
-			language = Language.getLanguage(languageTag);
-		}
+        language = Language.getLanguage(Utils.getLanguageCode(locale));
 	}
 
 	public static void saveConfig() {

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

@@ -6,12 +6,13 @@ import emu.grasscutter.Grasscutter;
 
 import javax.annotation.Nullable;
 import java.io.InputStream;
-import java.util.HashMap;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.Map;
 
 public final class Language {
     private final JsonObject languageData;
-    private final Map<String, String> cachedTranslations = new HashMap<>();
+    private final Map<String, String> cachedTranslations = new ConcurrentHashMap<>();
+    private static final Map<String, Language> cachedLanguages = new ConcurrentHashMap<>();
 
     /**
      * Creates a language instance from a code.
@@ -19,7 +20,13 @@ public final class Language {
      * @return A language instance.
      */
     public static Language getLanguage(String langCode) {
-        return new Language(langCode + ".json", Grasscutter.getConfig().DefaultLanguage.toLanguageTag() + ".json");
+        if (cachedLanguages.containsKey(langCode)) {
+            return cachedLanguages.get(langCode);
+        }
+
+        var languageInst = new Language(langCode + ".json", Utils.getLanguageCode(Grasscutter.getConfig().DefaultLanguage) + ".json");
+        cachedLanguages.put(langCode, languageInst);
+        return languageInst;
     }
 
     /**
@@ -42,6 +49,7 @@ public final class Language {
     /**
      * Reads a file and creates a language instance.
      * @param fileName The name of the language file.
+     * @param fallback The name of the fallback language file.
      */
     private Language(String fileName, String fallback) {
         @Nullable JsonObject languageData = null;

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());
+    }
+
 }