To find out what languages an app is available in, read the list of languages on its listing before installing it.
Open the app store (App Store on iPhone, Google Play on Android), tap the magnifying glass, type the name of the app, then tap its name. You should see its listing: the page that presents the app, with the Get or Install button.
Scroll down to Languages (iPhone) or read the description (Android). You should see the language you are looking for in the list.
If it is listed, the app follows the language of your phone.
If you don’t see it: open the app’s settings, then look for « Language ». Some apps offer the choice inside.
Limitation: a description in one language does not guarantee that everything inside is translated. Videos and documents sometimes stay in another language.
To picture it: it is like the instruction manual of a device: the box can be in your language without every page inside being so.