DOMParser errors leak locale

While writing a test for legacy/trac#30304 (moved) I found (yet) another way to get browser locale:

const doc = (new DOMParser).parseFromString('getyourlocale', 'application/xhtml+xml');
alert(doc.getElementsByTagName('parsererror')[0].firstChild.textContent);

test: https://acatarineu.github.io/fp/locale_domparser_error.html

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