The problem is solved now. With the help of the staff we were able to figure it out. We do not know the exact issue, but it was most likely something with the cookies on aliexpress. These did not recognize that the comments should be translated into German, instead they were translated into english. The problem might have been because I use two browsers (firefox+chrome), so if you run into this issue, make sure the language is set to your proper language in all the browsers. Furthermore, navigate in your browser manually to the subdomain of the aliexpress lanugage (eg. de.aliepress.com in my case). That solved it for me.