Update multilingual plugin

- User pcvars
- Remove vault usage
- Remove amx_setlang (not very useful, it's covered by the cvar now)
- Remove server language from the menu (covered by the cvar now)
- Add a cvar for previously hardcoded DISPLAY_MSG
Update core to use amx_language cvar instead of vault
Add extern amxmodx_language
Use MAX_PLAYERS
Use formatex and charsmax
Merge pull request #82 from xPaw/ml-plugin-update

Multilingual plugin update