Designing for the illiterate (or inebriated) is a nice goal, but designing for people to change their app into a language they cannot read, and then expecting it to all be easily usable, is not really a particularly great design goal. In that instance, a button could have a single word that 99.9% of people can read, but if you've changed it to Spanish it may render the button incomprehensible. I personally can't think of a single app I use that is completely usable without basic reading comprehension. I'd be curious about examples though (assuming reasonable complexity) because it'd be some potentially useful design that I'd be interested in seeing (and potentially using).
Adding to this, I'm curious how someone who cannot recognise the characters of their languages would use input fields, too? I don't think I know of many input fields which are particularly accessible if one doesn't recognise language characters without using speech recognition (which kind of sidesteps the issue).
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.
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.)
Adding to this, I'm curious how someone who cannot recognise the characters of their languages would use input fields, too? I don't think I know of many input fields which are particularly accessible if one doesn't recognise language characters without using speech recognition (which kind of sidesteps the issue).