Can the app translate to English?

Technically it is certainly possible to add different languages to an app, as is demonstrated by apps that cater for a wide international audience such as the Euronews app. However, Simyo being a low-cost telecom provider which specifically aims to serve cost conscious Dutch customers it will not be economically attractive to invest money in developing a multilingual website and app.

But you are not entirely out of luck, since there are people on this forum (moderators and some Simyo customers) who are fluent in English and who will be more than happy to help if you run into a problem and your grasp of the Dutch language is limited.