Use server language as fallback if client's "lang" key is not defined.

Fix a bug where if a client's ```lang``` info couldn't be retrieved (```config.cfg``` in read-only mode, or such key doesn't exist), the expected behavior is the fallback should be the server language defined with ```amx_language``` cvar ; not ```"en"```.

Change should be safe as, if for some reason ```amx_language``` is empty, it will use ```en``` at the very end.