Modern browsers start to tighten down on third-party access to cookies.
E.g. in current Firefox, a warning about the userLanguage cookie was
shown since it did not yet explicitly set the SameSite attribute and the
default is about to change.
The cookie name being referred to as BACKEND_LANGUAGE_COOKIE_NAME
suggests it should be readable by the actual Akkoma backend, which can
live at a different domain than akkoma-fe. Thus explicitly enable
sharing with third-party sites.
No warnings were shown for other cookies, so I assume
this was the only one not yet setting SameSite.
