Hi @h2f,
you haven't made any mistakes, I was the original translator for the Welcome app but I am not a native German speaker, in the meantime; there have been others which have been working on Welcome translations.
Welcome is specific to Ubuntu Mate and the mainstream translations are what you are talking about, sign into Launchpad and you can work on any missing German translations, I have done a few myself but there are more native German speakers on Launchpad so I am kind of in the way!. 
https://translations.launchpad.net/
As @lah7 has pointed out in his reply, there is still plenty of work for you!.
If you would like to help on this forum, here is where you can start by helping me (and the others!) to finish the beginners guide, anything you think of can be added to the guide once you have been on the forum for a while and your privileges are raised as to what you can or cannot do on the forum!: