UF's language codes do not always map to valid HTML lang codes

[alexweissman]

In https://github.com/userfrosting/UserFrosting/commit/0ba8ab4ce6bca724b07d118c90e3692cbf465520#diff-0ec1ca6d6c0abd16b8fdf4a504f04f77d102a96c10d53404405c2649a2bb8e4c we change the lang attribute of the top-level html tag in the core base template from a hardcoded value of en-US to a dynamic value pulled from the user's language/locale.

The problem is that our internal language codes are not always valid lang codes as per https://www.w3.org/2005/05/font-size-test/starhtml-test.html:

Notably, most of our localized codes are invalid because of the underscore. We should either change our naming schema to match RFC 3066 (which would be a breaking change) or find a way to define the corresponding lang codes. One possibility would simply to define them with a custom language key.

[lcharette]

 One possibility would simply to define them with a custom language key.

That would be the easier solution.

[Silic0nS0ldier]

Odd, I thought we were swapping the underscore for a dash. Something might have broke, or I'm remembering wrong. Custom language key would be the most reliable option I think.