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.
To upload designs, you'll need to enable LFS and have admin enable hashed storage. More information