Feature: #75806 - Add hreflang support

See Issue #75806

Description

"hreflang" tags are now added automatically for multilanguage websites based on the one-tree principle.

The href is relative as long as the domain is the same. If the domain differs the href becomes absolute. The x-default href is the first supported language. The value of "hreflang" is the one set in the new sites module.