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

SyncTime currently supports English and German. macOS will launch 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 Preferences, under General > Language & Region.

At the top you can change your preferred languages.

At the bottom you can select specific apps to use a specific language.