Better display names for language switching

Previously, we used CrowdIn's language name to display directly to the user.  However, two problems:
1. Those names are not properly capitalized (#1346)
2. Those names are all in English, meaning we'll need to translate them (#1019)

An easier option is just to show the "native name" we have stored for known languages, which (a) can be capitalized and (b) are in the target language, and so don't need to be translated.

I made those changes here, along with a few tweaks in supporting code.