Add lang tag helper to WYSIWYG

[alexdryden]

For works with multiple langues, we currently need to manually add language tags (<span lang="fr">...</span>) around anything that is not in English in order to meet accessibility standards. This creates a lot of opportunities for typographical errors through manual typing. It would be useful to have a feature similar to the "apply class" button that would apply a language attribute. The pattern could be identical:

(@angwatters @dgtracy for visibility)