Frontend language change should inform the backend

[olemartinorg]

Description

Language support was added in this PR, by having an app return all the supported languages to the frontend for the language selector to work. However, when the app language is changed in frontend, the backend is not informed about it in any way, which makes it impossible for validation messages etc to be adjusted.

Suggestion

• Add/set a cookie from frontend informing the backend about the language change. This should be validated against the available languages on the backend, and an internal API should expose it.
• Invoke ProcessDataRead again after changing the language, making it possible to manipulate the data model after language changes.

Additional Information

Discussion on slack

[StianVestli]

org/ssb