The text in CommCare applications is provided by each individual project, and many are built with translations for several different languages. You may use essentially any language. It looks like we have language codes built in for 487 languages, but please let us know if you'd like to use one that's not currently specified. You may be interested in this forum post for reference: How to Add New Languages to the APP which does not exist in the list
UI translations are available for many languages, but you may also provide translations if they aren't available for the languages you need to use. More on that here: User Interface Translations - CommCare Public - CommCare Public
CommCare HQ (the website) is primarily available in English, but most of it is also translated into French and Spanish.