The currently supported languages are German, French, and English. If your smartphone is set up to use one of these languages, the app will automatically adapt. If the device language is not yet supported, German will be the default app language. If you want to change the language in the app without changing the main language, you need to add one of the supported languages to your device's language preferences.
Go to your device settings > General > Language & Region.
Add the language you want to use to the list. The first supported language at the top of the list will be used in the avec app.
The process depends on the device model and operating system. Here you can find a well-illustrated guide on how to change your phone’s language preferences: https://www.wikihow.tech/Change-the-Language-in-Android