Is it possible to host the web-frontend for the forms via german servers (cloud) or only the data?

As far as I see, you have selected to store your data only in Europe(Germany). We have no restriction to use custom domains with JotForm according to the datacenter's locations.

EU Safe Forms: Our Solution to the EU Safe Harbor Invalidation

You can follow this guide about How to use Custom Domains with Jotform.

I am not sure whether I understood your question correctly. If you have any further problems, feel free to contact us again.

 Yes, ok it is possible to cname the jotform-domain with myown-domain. But in the background there are many request to san francisco.

So do you also host the complete form-builder on EU-Cloud(Germany) yet? If not, are you looking forward to do it in the near future?

Or do you have an on-premise or hybrid-hosting option?

Yes, we host the form-builder in EU-Cloud when we you use eu.jotform.com. 

Please let us know if you have further questions.