Add German translation

Thanks for your work on this truly amazing file manager :+1: Here is your German translation ;). There were some rather unusual English wordings I stumbled upon like the verbs with the -ing suffix (I'd rather say deleted or deleting file... ) or all your kinds of modes which may be a bit confusing and too much Capitalizing for my liking. Also, wouldn't it be easier if arrays.xml contained string references instead of hard-coded strings so one would only need to translate one file?

Maybe I'll correct some of it in another pull request when I find the time. Other than that, keep up the great work!