Add some sort of feedback if the selected spell checking language is actually installed.

Either make non installed language grey-out/non-selectable or give a warning/error if the selected language is not installed.

 

Currently selecting a language not installed on windows 10 gives no feedback and it's not clear that source tree uses languages from windows 10. Giving feedback if a language is not installed would improve user experience and possible saves user from creating bug reports because spell checking isn't working for them.