Ok, there are 4 language settings in Windows Phone: keyboard (K), language (L) + format (F), region (R), speech (S). I had everything set to DE except (L). Now, on your request, I set (L) + (R) to US and restarted. No change: everything runs perfectly.
After this I moved the language DE up above US-EN and restarted the phone. By this action (R) remained US as expected.
I clicked as usual in the Bluetooth settings on the band to connect it (not to have to wait too long) - and as expected and corresponding to all my tests with L=DE, R+F=US email notifications do not work. Setting R+F to DE did not help either.
Setting only L=US helped then, as all may test of the past months showed, at least on my phone (Lumia 930). And I heard the same from others.
Do you have any other idea? Are your notifications working with L not equal to US-EN?
I really have no problems having my phone's language set to English. However Cortana is not working with L=EN + R+S=DE, and that's really sad. "Not available in your region with the language you chose". With everything set to DE or everything to EN Cortana works, for sure.