D9 Language Switcher

This will need revisiting since SA-CORE-2023-003 broke the language switcher. It's possible that this could be resolved via a core regression fix though ( 🐛 [regression] Language switcher block throws exception when no route is matched Fixed ) so we could just wait on that. But my own client needs a fix ASAP in the meantime...

Here's an updated patch that should work since the core update.

Note that I have raised 🐛 [regression] Inaccessible language switcher links are removed before alternatives can be provided Fixed for core too though.

🐛 [regression] Inaccessible language switcher links are removed before alternatives can be provided Fixed has been committed to core, we should now revisit this will work as expected.