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/resources/languages/en-US.json

@@ -188,6 +188,11 @@
       "success": "Killed %s's current character.",
       "description": "Kills the players current character"
     },
+    "language": {
+      "current_language": "current language is %s",
+      "language_changed": "language changed to %s",
+      "description": "display or change current language"
+    },
     "list": {
       "success": "There are %s player(s) online:",
       "description": "List online players"