Invalid lang HTML Tag when "German (Germany)" locale is set

Steps to reproduce:

1. Login as Administrator
2. Open the top-right side bar and click "Administer Wiki"
3. Go to the "Content > Localization" section
4. Change the "Default Language" field to "German (Germany)".
5. Click save

Expected result:

The HTML 'lang' global attribute is set to 'de-DE' using a dash ('-') as a separator (see: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang#language_tag_syntax)

Actual result:
The HTML 'lang' global attribute is set to an invalid 'de_DE'. (See screenshot)