Set default dictionary according to locale in the browser

The fix for fix default iceweasel spelling language (#5371 (closed)) (using the amnesia branding extension so that it sets the preferred dictionary according to the current locale) does not really work that well. For most languages we testing on 0.17~rc1, no dictionary was selected by default. English had one, though.

Related issues

Related to #9021 (closed)
Related to #10272 (closed)
Related to #15807 (closed)
Related to #16571
Has duplicate #6587 (closed)

Original created by @tails on 5962 (Redmine)

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