Locale-dependent JS methods may leak language info to content window
String.localeCompare, Number.toLocaleString, and Date.toLocaleString may leak locale information to the content Window, even if the user is spoofing English.
See also https://bugzilla.mozilla.org/show_bug.cgi?id=769871 and ECMA 402 (http://www.ecma-international.org/ecma-402/1.0/).