For now it seems to be tied to your phones language. You are not the only one making this request so hopefully we get a language option in the app for the future(bonus points to Google if they make it an option for individual games and even more points if we can select different voice and sub title languages as well).
I would looove the option to switch to English in Destiny 2. It would make following guides and online videos so much easier for me. While I'm sure they did a decent job of translation, an option to chose from would make *so* much sense!