update hreflang locale validation

[kamilkrzyskow]

Fixes #193 Re-commit of the core changes in https://github.com/ultrabug/mkdocs-static-i18n/pull/197 with a few minor additions (removal of unused variables, warning message update).

Co-authored-by: @jonaharagon Jonah Aragon jonah@triplebit.net Co-authored-by: Hugo Slepicka hhslepicka@users.noreply.github.com

[ultrabug]

Thank you