Okay... so I must admit, I'm a bit baffled. What you are describing is functionally impossible. The clients cannot get the en-US binary if it's not been published.
changed Download URL to German Firefox installer location
This step may be problematic. In fact, after downloading the German binary, you should have selected the option "I already have the content..." and configured that option.
When you published your modified (German-language) package, did it download the German-language binary prior to publishing it to the WSUS server?