Add Language API

[MelnCat]

This pull request adds a Language API to translate translation keys and Translatables into the default server language, along with checking if a translation key can be translated. This is useful for cases where it isn't possible to use text components, but you still need a Translatable turned into human readable text.
Without this API, to translate translation keys you would need to either use NMS, or use the adventure plain text serializer. However, the plain text serializer needs wrapping the translation key into a TranslatableComponent then running the serializer on it, while having this API would make it a simple method call which goes to an ImmutableMap access. It was also not possible to check if a given translation key could be translated with the plain text serializer.