They translated it once years ago, likely using external resources. My team has had this issue; we made internationalized versions of our website once, years ago, and it was quite expensive and used external language contractors. Then the i18n versions gradually fell out of date as we continually updated the English version (because that's what we full-time devs speak). Eventually, years later, the i18n versions were hopelessly out of date and turned down, because it wasn't worth paying for another round of internationalization on them given how little it turns out they were used.

Maybe they fully translated an earlier version of the app but more strings were added in subsequent versions and not translated. When there's no translation available for a string, it falls back to English.

Because either you have to pay some native a decent money to make a translation or because some strings are embedded in code?