Localization: EN + DE

[ArchibaldBienetre]

Implements #34

• [X] extract UI translations
• [x] implement language switching of the UI
  • this requires some research...
  • e.g. a bound property for the label text may be an option https://stackoverflow.com/a/19826636/1143126
  • or, something else https://edencoding.com/dependency-injection/#creating-a-resourcebundle
• [x] extract translations singleton / static class (as lightweight alternative to dependency injection)
• [x] implement language switching for back end code (e.g. exceptions, maybe even logging)
• [x] as usual, tests...

[ArchibaldBienetre]

Not ready yet, though

[ArchibaldBienetre]

@DonMischo please have another look, give it a try locally, then hit merge! :)