Use `classmethod` as a decorator.
Fixed #21469 -- Allow set objects in Meta.unique_together.

Thanks to Tim for the review.
Fixed #21458 -- Made check_for_language more resistant to malformed input.

Thanks to Sergey Sorokin for the report and to Bouke Haarsma for the review.
Fixed #21472 -- Fixed inline formsets display when parent pk is 0

Thanks agale031176@gmail.com for the report.