@fokusferit @fragsalat @tonysaad and me decided to go with following approach for internationalisation:

Each component that uses localisable labels has a property lang that takes in a language identifier.
The language can be chosen in the WS-Header. There is an language-changed-event propagated by the WS-Header the dev can listen on.
The localised labels are stored in a json per component.
The currency- and date-format are affected by this language property.
The default is always german ('de').
Both currency- and date-format can be overwritten by the properties currency-format and date-format.