Add German as invoice language

NL, FR and EN are supported, but it would be great to send the entire invoice in German. Some of my customers are in Germany, and English can be difficult, especially when payment position descriptions contain technical language. Also German is one of the official languages in Belgium, albeit practically only used in the duitstalige gemeenschap. Currently I am using the English language setting and writing my position descriptions in German so my customers understand what they are paying for. It would be helpful for them (and their tax advisors) if everything else would be in German, too. I am offering translation assistance, except for the legal aspects (terms & conditions).