Off hand I can think of a few reasons for using the mapping object as they did in angular-translate:

- It may make sense to translate the same English sentence to something slightly different in different contexts in German.

- JSLint/JSHint should be able to check for usages on an identifier like TITLE, PARAGRAPH_ABOUT_FOO, etc. easier than comparing strings (you'd have to build something custom for that).

- Brevity. If you have the same long string of text written twice it may be useful to have a shorter identifier for it.

I agree with not translating language names though, in the real world that doesn't really make sense.