Although English is the dominant language and can be used while proving your concept, once you start to scale and open your app in different countries, it will start to be a friction point. As you can imagine not everybody is comfortable with English on a daily basis and it can be especially frustrating if they are faced with a problem in the app. Trying to discover a feature, making the app accomplish what it is supposed to do in a foreign language and not succeeding can even lead to a bad review.
This is why it is important to localize as much as you can as you scale up. Fortunately, you don’t have to this manually, once the translations are done there are many tools that enable to localize the content of the app automatically. Lokalise is one of the best out there but you can easily find other alternatives as well.