[another-dave]

As a (hypothetical) example, I'd imagine a pizza app to be pretty usable if I didn't speak the language (illiterate may be a bit much as I'd need to know my address):

Put in my postcode & choose my address; choose a pizza size & toppings from icons/photos.

Add credit card info into a standard looking form & that's it (or even, touch the fingerprint sensor when the fingerprint icon comes up)

I think the trouble isn't that you wouldn't know the Spanish for "OK" (or "pepperoni") but if the app lacks proper information hierarchy so you don't know what to do next.

[Leherenn]

If you're a foreigner you would probably be tripped by the post code/address. I certainly was when the petrol station asked for my post code in the US! (My card postcode does not fit the US format.)