Switch language in navbar from ISO codes to actual language name

[misaugstad]

Brief description of problem/feature

I believe that we used ISO codes to save space in the navbar, but at this point it's probably worth it to use the actual language names, especially since we've included Chinese that doesn't even use the same script!
en-US: US English
en-NZ: NZ English
en: English
es: Español
nl: Nederlandse
de: Deutsch
zh-TW: 繁體中文

It probably makes the most sense not to translate the language names. If you're looking for a specific language, you probably can read in that language and know it's name :)

[misaugstad]

I think that this should be pretty easy, and I don't foresee any breaking changes by doing this.

First, add text for each of these in the conf/messages file (the conf/messages.* files are for specific languages and they override conf/messages, but conf/messages matches any language).

lang.name.en-US = US English
lang.name.en-NZ = NZ English
...

Then get the text in navbar.scala.html by replacing @language with @Messages(s"lang.name.$language").