Update strings.xml

Added all missing strings from /app/src/main/res/values/strings.xml, fixed some grammar/punctuation and changed some bits to make it sound either more natural or more close to the original (English) version. I also added consistency as the same terms were translated differently throughout the file. I even found a (hopefully) good German replacement for the nobby-no-mates joke. ;)

Note to Astonex: The German translation uses the informal form of address "Du". This is used between friends, colleagues and  so on. If you want a more formal way to address the user I can change it to "Sie". This is used for people you don't know personally, respectabilities, elders, etc.