Depending on whether you use Android or iOS, there are different ways to change the language of your Tomorrow app. 

For Android: In the Tomorrow app, navigate to 'Manage App' via your profile and select 'Language'. You can choose between German, English or device language. If your device language differs from English or German, the app will automatically use English as default language. 

For iOS: First go to your device settings, select 'Tomorrow' and then 'Language'. There you can choose between English and German. The default language for the app will be your system language. If only one language is displayed, this is because you only defined one system language. To define multiple languages, head to your decive settings and select 'General' - 'Language & Region'.