I checked and you're right, but I could only reproduce it on the test page you have created. And the builder is not loading because of redirect, try accessing this
/de/test-german/ and it will redirect back to home page. And with that, the builder will not really load.
And it's all because the German language is hidden, there is a bit of confusion since I was testing the Swedish language (I only saw two languages English and Swedish).
I went ahead and toggle off the option that hides it, and the redirect doesn't happen anymore and so the builder works.
It's in Admin > WPML > Languages.