fix default iceweasel spelling language

Logged in with the default (English) settings.

Visited an English speaking open-publishing website (http://atlanta.indymedia.org/), started to publish => dictionary is automatically set to French (Luxembourg) in the main textarea.

Looking around on the web, it looks like it’s expected that iceweasel picks any seemingly random dictionary as the default one.

Implemented in bugfix/fix_default_iceweasel_spelling_language, done in Tails 0.17.

But the fix is not perfect, see select localized spelling dictionary in Iceweasel.

Original created by @tails on 5371 (Redmine)