Invalid lang attribute on head element (should be en-GB, not en_GB) #8180
Description
Currently the head tag for en-gb users is:
<html class="ng-csp" data-placeholder-focus="false" lang="en_GB">
The lang attribute's value is invalid, it should be en-gb (with a dash), not en_gb (with an underscore).
See more at this page: https://www.w3.org/TR/selectors-3/#attribute-selectors
Currently Nextcloud is showing this as an error in the Google Lighthouse Accessibility audit - so would be good to fix.