locale filter doesn't always return the language

Due commit ffb11655a0db73534be23c73b24f988f1184a01b things got broken because of interface_lang vs content_lang. in class-locale.php you see cases where the is_admin() check is commented.

Current situation is that language_attributes() returns the incorrect language code because it's setting the content_lang but only interface_lang is returned.

Start in class-locale.php and trace the language_attributes() path, especially the cases where the is_admin() check is commented out. Compare how content_lang and interface_lang are assigned and returned. Done means language_attributes() returns the expected language code in the affected contexts.

Written by the indexing model from the issue text.