Indicate if selected translation came from Accept-Language header in language switcher. #12444
Description
What's the problem this feature will solve?
With support for Accept-Language (ref: #6864) launched in pypi/infra#109, there is now implicit functionality in how the translation served from pypi.org is selected, while it can be explicitly overridden by clicking a specific translation in the switcher.
This may cause some confusion for users. We should help them by indicating if the translation was selected implicitly.
Describe the solution you'd like
An indicator somewhere in the language switcher in footer that notes if the translation chosen was due to the Accept-Language header rather than the _LOCALE_ cookie. This should link to some reference or our own help entry to how Accept-Language works.