Hi, Joost here, founder & CEO of Yoast.

I can read German quite well but typing is hard so please excuse me for commenting in English. I’d like to say two things:

First of all: Yoast SEO has import functionality for wpSEO.de and we fully support German in all our analyses. If you want a plugin as lean as wpSEO.de, you can simply disable all the functionality in Yoast SEO you don’t like. We’d love to hear what you can’t do with Yoast SEO that you can do with wpSEO.de. Feel free to reach out to me on Twitter (@jdevalk) with questions and comments.

Second: plugins should be GPL. The WordPress community at large is quite clear about this. Not adhering to the GPL will mean that you’re banned from WordCamps and basically can’t operate in this space very well. Choosing to do that is in my eyes not a good idea and also dishonest: you’re keeping something for yourself while using all of WordPress that was provided to you for free.