Support more languages

[bminixhofer]

More languages were understandably already requested multiple times (e. g. #14). This issue tracks progress in this area.

The primary goal is to make it easy for contributors to add a language, not necessarily to support many languages right away.

I have decided to not add support for any new languages until Spellchecking (#2) lands and the quality issue in the core (#44) is resolved.

Theoretically someone could contribute a new language right now but since LanguageTool has language-specific code there will likely have to be some adjustments in the core for each new language which is not easy enough with the current level of documentation. So for adding new languages to scale well the quality of the core has to be improved significantly.