gettext.c: move get_preferred_languages() from http.c

gettext.c: move get_preferred_languages() from http.c

Calling setlocale(LC_MESSAGES, ...) directly from http.c, without
including <locale.h>, was causing compilation warnings.  Move the
helper function to gettext.c that already includes the header and
where locale-related issues are handled.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>