As per checking, with your plan, we've got you covered in Germany!

Data and texts are unlimited and free. For making and receiving calls though while in Germany its $0.25/min

You can use this link as reference too:

https://www.t-mobile.com/support/coverage/international-roaming-services#heading1


When you arrive in Germany, you can dial #766# to turn on international data roaming and to turn it off its #763#