[Suggestion] Implement Multi-language support

[LightDestory]

Multi-language support has been removed on the fork on which AzerothCoreAdmin is derived. We should check on older TrinityAdmin versions and retrieve the multi-language support, adapting the feature with the current code design.

[LightDestory]

After a fast look up I noticied that the feature is there, but with disabled language. It can be easily enabled but it misses a important feature: fallback of missing texts.

I will look into it later, but for now... You can add your language easily.