Beste antwoord door David
Even though this topic has been closed we've noticed that this has been the most Googled topic regarding this question, so a quick update!
The website is still only available in Dutch, but the My T-Mobile App can be switched to English! Go to the My T-Mobile App and open the menu (3 horizontal stripes) and go to 'App instellingen'. Here you click on 'Taal' and press 'Engels'. Your app is now fully English!