Update: All the problems reported in this post have been fixed.

Issue #1: Languages with country cannot be used (Fixed):

https://github.com/unacms/una/issues/4651

Issue #2: HTML lang tag value (Fixed):

https://github.com/unacms/una/commit/9cb5d6bb40d08df81aaf37661cec8f9214864cb9

Thanks a lot to @Anton L who investigated those problems and provided for the solution. Thanks to UNA Team for the great work you guys are doing.