Different app language from system language I maybe am wanting the impossible 
I want the system to speak Dutch  but also that the apps stay speaking english

I can change the system language to Dutch but then many apps also start speaking Dutch and that I don't want

The idea behind is just a very crude safety measure at startup as non Dutch speaker you are lost so the number of potential users is severely restricted :)

While I can use English for the rest 

But is this possible?
I don't think you can, unless it's a rooted phone. Apps will always be shown in the system language if a translation is available. English is used as a last resort. Not to mention this is a very poor security measure.

Root option: install Xposed, look for a module named App Locale.