macOS launches an app in the first of your preferred languages that is also supported by the app, e.g. if German is placed higher in the list than English, the app will be launched in German. You can change this in System Settings, under General > Language & Region.

At the top you can change your preferred languages.

If you want a specific app to be launched in a specific language, scroll to the bottom, add the app to the Applications list and specify the desired language.