The language setting of the app depends on your phone settings. The app looks at the priority list and selects the first option available to the app. This means that if the app, for example. is in Japanese, Japanese is ranked with higher priority than English or German. You can change that by following these steps:
Apple:
Go to your phone settings and search by language and region.
Select the desired language under Language and Region and adjust the sorting if necessary (e.g. sort English higher than Japanese).
Save the settings and restart the ISIC app.
Android:
Go to your phone settings and search for language and input.
Select Language and make sure that your desired language is at the top of the sort order (e.g. sort English higher than Japanese).
Save the settings and restart the ISIC app.