Tor Browser 8 always prompts wrt. asking webpages in English

Originally created by @intrigeri on #15706 (Redmine)

… when started in a non-English locale. That is, extensions.torbutton.prompted_language is not honored.

Feature Branch: feature/15803-tor-browser-8.0a10+force-all-tests

Edited May 15, 2020 by intrigeri

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