Question
“How can we get German colleagues to list upfront all of their concerns before saying no?”
Answer
Perhaps you should anticipate their concerns, then build your arguments into your request, thereby demonstrating that you have done your homework, and that you have taken their perspective into consideration.
Secondly, perhaps you should not confront Germans with yes-no questions, to which you expect them to respond with a yes immediately, without them having time to understand the request, and to do some serious reflecting on whether they should agree with it or disagree.
In other words, there are alternative ways of introducing your request, maybe as a thought, as a suggestion, as something to collaborate with them on.