Thank you for your answer ... but read the full summary next time please...
For other people that have the same problem, I've figured it out by now:
1) The language you select in the creative cloud packager is not for the Creative Cloud App, only for the applications you package
2) The Creative Cloud App Language is set by the language of the OS ... this has nothing to do with the Creative Cloud Packager
3) When the Creative Cloud App starts in a specific language ... only the packages installed for this language are detected
4) If you want to change the language of the Creative Cloud App and you had packages installed under the previous language, you have to download (or install) them again under the current language to be visible / updatable again under the current language
For me this means:
option1: I package all the applications in dutch (since all CC for teams applications are multilanguage)
option2: I notify all users (or use active setup or any other alternative) to manually change the language to english ... otherwise no packages will be visible