Ensure the lang-fixup redirects set a Vary header for Accept-Language #14500
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Previously, we relied on similar code deep in lib.l10n_utils.render, but BedrockLangFixupMiddleware runs way before then, so we need(ed) to add the Vary header here, too.
The orginal code in l10n_utils.render still has value, because it handles the fallback for a missing/unactive language to en-US
Issue / Bugzilla link
Resolves #14498
Testing
Check the headers when you go to / and it redirects to /en-US/ (or whatever your browser's local language is)