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).