Browser locale can be obtained via DTD strings

See https://bugzilla.mozilla.org/show_bug.cgi?id=467035.

Works in Tor Browser and Firefox 67 (with a different dtd file as in the bugzilla PoC), probably also next ESR.

Did not do a PoC but it would be easy to get a specific string in all locales, and just compare with value obtained via a hidden iframe that loads an xml with the translated string.

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