www-client/firefox and friends should use L10N instead of LINGUAS to control downloading and installing of language packs.
Note that the following language tags are considered duplicates. Ebuilds should use the generic language tag without the region subtag for these, and remap to upstream usage where necessary.
# fy-NL - Western Frisian (Netherlands)
# ga-IE - Irish (Ireland)
# gu-IN - Gujarati (India)
# hi-IN - Hindi (India)
# hy-AM - Armenian (Armenia)
# nb-NO - Norwegian Bokmål (Norway)
# nn-NO - Norwegian Nynorsk (Norway)
# pa-IN - Punjabi (India)
# sv-SE - Swedish (Sweden)
Taken care of in mozlinguas-v2.eclass -- packages will migrate as they are released; latest stable(-candiate) and ~arch packages for www-client/firefox and mail-client/thunderbird have already been migrated.
Marked the bug as in-progress for now until mozlinguas.eclass is ready for removal, feel free to close it though if waiting is unnecessary to resulve the tracker.
mozlinguas.eclass is gone since almost a year. Closing.