Changes to `intl.accept.languages` get overwritten after restart

After mentioning an issue with localized content on the blog ( https://blog.torproject.org/comment/269161#comment-269161) I investigated a bit and it seems changes to intl.accept_languages get overwritten after restarting Tor Browser by our custom values. Other preferences might be affected as well.

This is at least affecting our fix for legacy/trac#21999 (moved).