Sorry but I don't understand how does the first sentence in your post relates to the second one.
To me it reads like you didn't know the local language but they're the illiterate ones for not knowing your language? Or did I completely misunderstood your story?
Are they measuring "reading ability for the real world" or are we talking "analyze any interpret fiction literature" type reading skills? Serious question. Because I feel like "real world reading" stopped after about 8th, maybe 9th grade and everything else was just fluff.
Google Translate is really, really, really bad for Vietnamese, so they were probably having trouble understanding the mutilated Vietnamese it was spitting out, more than anything.
I assume you didn't mean to imply that "basic literacy" == "knowing English", but your post does somewhat come across that way.
Made me realize there are still big chunks of the world where you can't take basic literacy for granted in 2023, even of people in their 30s.