Sadly, the support team of this group suggested me the only (bad practise!) way to do that (for some reason, as private mail instead of public post):
The form itself is nothing else than a HTML5 file with hardcoded strings inside tags rendered inside a WebView. You have to edit these strings.
I needed multilanguage support too, so I added multiple copies of this HTML5 file (assets/consentform.html) and translated them to get one form for each language.
Then I edited the code inside ConsentForm.java around webView.loadUrl() to get the multilanguage support by loading the one matching the needed language.
Attention: Inside the HTML5 file are two different forms: The big one supporting consent for personalized ads and a smaller one that appears, if only non-personalized ad consent is requested.
I didn't have time anymore to wait for something more useful so I only see this as a crappy compromise solution in the hope that there will be a better Consent SDK in the future (without the need to hack the source code of a lib to meet most basic needs like language support ).
PS: It would be nice if the small form does support the option of redirecting to the paid version too like the big one does. Furthermore, the big form could have scrolling support because it appears truncated on one of my devices if the font size is set to big in accessibility settings.