Tell Firefox to set language setting to English (refs: #15706)

This probably won't be enough to fix #15706 ("Tor Browser 8 always prompts wrt.
asking webpages in English") until
https://trac.torproject.org/projects/tor/ticket/26409 is fixed.
But once Torbutton is fixed, then this should avoid Firefox prompting
the user.

config/chroot_local-includes/usr/share/tails/tor-browser-prefs.js

@@ -13,6 +13,10 @@ pref("extensions.torbutton.show_slider_notification", false);
 pref("app.update.enabled", false);
 
 // Suppress prompt and always spoof useragent as English
+pref("privacy.spoof_english", 2);
+// XXX: Now handled by privacy.spoof_english; remove the next two
+// lines once Tor Button stops doing it itself in addition to what
+// Firefox does.
 pref("extensions.torbutton.spoof_english", true);
 pref("extensions.torbutton.prompted_language", true);