[Feature] Localise/Allow multiple translations for OAuth button, Welcome message and system Emails

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request.

• Yes

The feature

While Immich has translation/localisation option for almost everything, it would be great to have the possibility for the administrators to localise:
- The OAuth button
- The Welcome message
- Emails sent by the system

Maybe it would be possible to upload a file of localisation specifying certain languages (or all) with a fallback to a specified default (or to Immich’s default if none is provided).

It is already possible to use a Template for the emails, but it does not solve the localisation issue. This might also require a way to choose the language when sending the email

This has already been requested in a certain way in #9920 and #18459.

Platform

• Server
• Web
• Mobile