It detects en-GB, de-DE properly, if checked from dropdown - but the issue is that for example polish language does not have any other options like those above and is not detected - probably, because translation plugins like WPML and Polylang(just checked on both) is using "pl-PL" and not only "pl"(like the value from option in dropdown) and core Wordpress itself too btw.
Changing <html lang> is not always possibile and by semantics not really recommended.
Also by those you can see that wordpress itself is not really using "en" as locale code anywhere, so it might be good idea to switch to "en-GB" for default locale.
It's regarding thing i have mentioned in forums: https://wordpress.org/support/topic/language-detection-5/
It detects en-GB, de-DE properly, if checked from dropdown - but the issue is that for example polish language does not have any other options like those above and is not detected - probably, because translation plugins like WPML and Polylang(just checked on both) is using "pl-PL" and not only "pl"(like the value from option in dropdown) and core Wordpress itself too btw.
Changing
<html lang>is not always possibile and by semantics not really recommended.Here you can check full list of locales codes: https://gist.github.com/danielbachhuber/14af08c5faac07d5c0c182eb66b19b3e
or here, more officially: https://translate.wordpress.org/
Also by those you can see that wordpress itself is not really using "en" as locale code anywhere, so it might be good idea to switch to "en-GB" for default locale.
Hope it's doable!