What should I do if an app won’t change language after reinstalling?

I downloaded an app abroad but didn’t use it because the language was in Portuguese and I couldn’t get it to revert to English. Now back home in the US, I deleted the app and downloaded again from the US App Store but when I open it it still is in Portuguese and won’t let me sign in. I’ve repeated this three or four times, each time restarting my iPhone but it’s still stuck with the data from the Portuguese download. Obviously deleting the app hasn’t got rid of all the data. So how do I get rid of this data that’s now contaminating my US app?

[Re-Titled by Moderator]