Optional Accept-Language support for the login page language

ideas
1

The login and identity pages take their language only from the origamCurrentLocale cookie; otherwise LanguageConfig.Default applies. A client that cannot set that cookie — a bundled frontend on another origin, an embedded browser — shows the default language until the user switches it by hand, although the browser sends Accept-Language.

Proposal: an opt-in key under LanguageConfig, off by default.

  • Cookie first, then Accept-Language with parent-culture fallback (en-US → en), then the default culture.
  • The cultures offered on the login page stay separate from the ones used for matching, so a fallback culture does not add a duplicate link.
  • Server-side localization in /api/private uses the same resolution instead of reading the cookie provider directly.
  • Template entry, the appsettings.json documentation article, and tests for the header and for the flag being off.

I can prepare the pull request; the result will be linked here.

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