Add German translation

[pille1842]

Add a German translation file. Fix #714 .

[martinheidegger]

Hello @pille1842 and thank you for your work on this! Did you notice that each exercise can be translated as well?

In every exercise folder, like in exercises/baby_steps, there are problem descriptions with the pattern problem.{lang}.md. Adding a problem.de.md would translate the exercise content.

Do you think this partial translation of some of the UI is a sufficient addition or do you want to also add translation of the exercise files?

[lukasjoc]

@pille1842 are you currently working on this?

additionally to adding all the exercise translations, the UI translation should also be used by adding the code de in learnyounode.js

I also noticed that the german UI translation may also need:

credits

"menu": {
    "credits": "CREDITS"
}

and a subtitle

    "subtitle": "Selektiere eine Übung und drücke Enter um zu beginnen."