From what I see there's no way to do so. The only option I could think of would be backing up all your setup folder, installing German version and overriding all but language files in hopes that they transfer the license, but tbh imo it's not really worth it. First, it's a risk it wouldn't work still without a key or would corrupt a program and need another reinstall either way; and second, while I understand the idea of running in language you prefer, you hit a roadblock every single time you google a solution to your question, because some things are translated in very weird ways, and finding your way around the mismatched translation makes it harder to follow tutorials and guides.

What I can tell you is to ask your sister to search for e-mails titled "Delivery information" - that's how both of my keys came when bought from EU/NA website.