The server by default sets the xml:lang and lang attributes of the <html> element to the the value of $ENV{LANGUAGE}. The LANGUAGE environment variable contains a colon separated list of POSIX locales. The xml:lang and lang attributes contain a BCP47 language code (aka an IETF language tag).
It isn't clear to me how different POSIX and BCP47 language codes are, but xml:lang and lang certainly do not allow a colon separated list. Considering that usually the user running the server is not the user interacting with the software, I think the musicbrainz server should ignore the LANGUAGE environment variable set by the user entirely.
Hm? On musicbrainz.org I see xml:lang="en" lang="en" and on i18n.mbsandbox.org I see xml:lang="en_AQ" lang="en_AQ", no colons (or semicolons) anywhere...
AFAIK the main difference between the POSIX and BCP 47 ones (for our needs) is that the former use underscores where the latter uses a dash, i.e. en_AQ should be en-AQ - http://www.w3.org/International/articles/language-tags/