Beste antwoord door Skyrider
I'm afraid that only Dutch is currently available in the T-Mobile App. I saw on multiple topics that they've forwarded the suggestions made by other people to include multiple languages into the app, but thus far I've never seen the ability to do so.
I apologize though, and I do understand the need for other languages. Maybe T-Mobile should release a platform so other people can make free translation contributions towards the app.