This converts the language button into a mw-ui-button which is the
class we're currently using in both Vector and Minerva. In future
this can become a WVUI button using the same roadmap as Vector
and Minerva.
This fixes the issue with the icon resize, as well as the issue
with the button lacking appropriate hover/focus states.
Bug: T289630
Change-Id: I8242986e8762a8f663f8cab07c54481e14223320