German translation update

added 1 new string, edited some strings to make it more serious (no
anglicisms etc.), some lapses in style, and edited some swiss-german to
german-german, no important updates, but it make it more qualitative

we should save the contributor, translator, etc. names just in one file and translate just things like the language names. by the way, I use this app nearly every day! thanks a lot for the development! May Allah reward you immensely!