Why is the app interface using [this language] instead of [other language]?

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 the System Settings, under General > Language & Region.

At the top you can change your preferred languages.

If you want an app to be launched in a specific language by ignoring your preferred languages, you can add it at the bottom and specify the desired language.