On the firefox-download website, http://www.mozilla.org/products/firefox/all.html many languages are listed, however in the present ebuilds there is no LINGUAS support. I am not aware if language packs can be installed separately (thus installing multiple languages), that would be nice. Some instructions are provided here: http://www.mozilla.org/projects/firefox/l10n/ Until then, would it be possible to use the first element in LINGUAS to decide which l10n to use? Yadda-yadda-yadda, I could have made the ebuild myself, but it looks intricate enough and I don't want to mess with things I don't understand much of.
I second your petition.
*** This bug has been marked as a duplicate of 70820 ***