This feature was implemented in b01544d690 to replace the session's language
in the request. 44ddbdf925 moved the process from the request to the response,
which made this feature unusable (because the request was already processed).
44ddbdf925 also made this feature set the language cookie. However, it is
overwritten by user preferences.
To fix this, we could overwrite the cookie of the response after it's set from
user preferences. However, it is not an ideal solution because when users
switch between Sites with different languages, the first response will use the
language of the previous page. Therefore, this ignores user preferences and
alters the cookie of a request instead.
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