4500: RFP warn if language set to non en-US

fixes #304

user.js

@@ -1587,6 +1587,7 @@ user_pref("privacy.firstparty.isolate.restrict_opener_access", true);
       This blocks exposure of local IP Addresses via mDNS (Multicast DNS)
  **  967895 - enable site permission prompt before allowing canvas data extraction (FF58+)
  ** 1372073 - spoof/block fingerprinting in MediaDevices API (FF59+)
+ ** 1039069 - warn when language prefs are set to non en-US (see 0207, 0208) (FF59+)
 ***/
 user_pref("_user.js.parrot", "4500 syntax error: the parrot's popped 'is clogs");
 /* 4501: enable privacy.resistFingerprinting (FF41+)