Language not spoofed for en-CA and en-UK
If a user installs an en-US tor-browser and then changes it to English (Canada) or English (United Kingdom) then neither the "locale spoofing prompt" is shown nor Accept-Language is spoofed (e.g. for Canada it's en-CA,en-US;q=0.7,en;q=0.3). The prompt is not shown because of https://searchfox.org/mozilla-central/source/toolkit/components/resistfingerprinting/RFPHelper.jsm#196. Besides, given javascript.use_us_english_locale=true will not be set, I guess the non-spoofed locale will also be accessible via some JS APIs.
I'm not sure about the right solution, though. Should we show the spoofing prompt for these cases too? Or should we always spoof these two cases by default?