[FEATURE] Use OIDC standard user claim member "locale" within the SSO flow and to set the user's default language

[menturion]

Use OIDC standard user claim member locale ( [ISO639‑1] language code, e.g. en-EN; see https://openid.net/specs/openid-connect-core-1_0.html#UserInfo) ...

1. to choose the user's language within the SSO flow (affected template: synapse/res/templates/sso_redirect_confirm.html) and
2. to set the user's default language for matrix clients (e.g. Element Web).

Currently, ...

1. the SSO redirect template uses en-EN only and
2. the language is chosen on the basis of the browser language.

[clokep]

See also #9386.

[menturion]

See also #9386.

The feature request filed here #9386 requests the support of multi-languages for the SSO flow.

The feature request filed here, however, suggests to use the OIDC claim member "locale" for two purposes:

1. to select the language based on the user claim "locale" for the SSO flow as the user currently uses for the IdP so that we have a consistent language along the flow and
2. to set the users's default language for synapse resp. the client also based on the given OIDC claim member "locale".

[callahad]

This sounds like a behavior we'd want to support (maintaining a consistent language as you bounce from Synapse to the IdP and back), but actually implementing this in a way that is usable and robust is a bit of a daunting challenge.

Are system / browser Accept-Language settings insufficient?

[menturion]

Are system / browser Accept-Language settings insufficient?

It is at least sub-optimal with respect to the UX. The "locale" user claim is the best hint to what language the user wants to use for the OIDC client which can differ from the system / browser Accept-Language setting. Thus, just using the system / browser Accept-Language setting will not ensure a consistent language.