Add German language category

This commit adds German as a selectable category on Planet KDE so German blogs/feeds can be added.

These changes were not tested so let me know if there is more to this and I need to dig deeper. cc @phunh

My motivation: I published a cool video log in German with English subtitles. I want to embed this video in a blog post that is supposed to show up on Planet KDE. I plan to use a German blog to publish it in this new German category and an English blog to publish it in the default English category.