Between thunderbird 3.0.4 and 3.0.5, the LANGS was split over several lines. Unfortunately, fy-NL is at the start of a new line, so LANGS no longer matches *\ fy-*. Because of that, LINGUAS=fy stopped working. To fix this, simply change LANGS="af ar be bg bn-BD ca cs da de el en en-GB en-US es-AR es-ES et eu fi fr fy-NL ga-IE he hu id is it ja ko lt nb-NO nl nn-NO pa-IN pl pt-BR pt-PT ro ru si sk sl sq sv-SE tr uk zh-CN zh-TW" to LANGS="af ar be bg bn-BD ca cs da de el en en-GB en-US es-AR es-ES et eu fi fr \ fy-NL ga-IE he hu id is it ja ko lt nb-NO nl nn-NO pa-IN pl pt-BR pt-PT ro ru si \ sk sl sq sv-SE tr uk zh-CN zh-TW" using backslashes to make sure no newlines end up in LANGS.
Thanks for the report I have just finished fixing it in the tree.
