Language override behavior changed upstream in WebKit/WebKit@039ebd9
New logic is closer to the actual behavior of WebKit on macOS, meaning that when the user changes system language the actual locale changes according to some weird OS rules:
ru-RU => navigator.language === 'ru'
fr-CH => navigator.language === 'fr-FR'
es-MX => navigator.language === 'es-MX'
Our locale emulation is aligned with that, so setting locale to fr-CH will result in fr-FR etc.
Max Schmitt