Frontend: i18n

[MrSnyder]

The frontend needs to have internationalization (currently English and German). As I see it, this involves two levels:

1. Backend should provide the OpenAPI-schema in different languages (depending on a query parameter), so that data-driven frontend components (e.g. the tables) can directly get strings in the active language from here.
2. Frontend-only strings.

For the 2. level, I suggest to go with i18next, as it appears to be the state-of-the-art approach for React apps.