Within installed PWAs, the navigator.language / navigator.languages property return a value, that is not according to the Safari settings and defers from the navigator.language / navigator.languages in the browser itself.
Example: Safari settings set to "German", returns "de-DE" in browser mode, but within the installed webApp it returns "en-GB".
I thought this might be due to the overall phone language settings, but this is not the case. Even is the phone is completely set to German, the navigator returns "en-GB" within an installed PWA.
My phone is also not English/British, there is no reason why "en-GB" should show up.