Textbooks by nature have to limit the variety of language a bit, just because, at least at lower levels, you can’t fit in too many options. It’s very easy to hear which structures, sentences and phrases are in a given country’s textbooks and are practiced, once you talk to a number of people from that country.
If you ask an unmarried Vietnamese if he or she is married, the immediate answer is almost always, “No, I’m still single,” with the word “still” drawn out. I assume this sentence is in their textbooks, because almost everyone from Vietnam says it. People from other countries have their own standard, drilled textbook sentences that are easy to detect.
In my first year of speaking Czech, it was pointed out to me that I used correct but unusual words for “usually” and other terms in that language. The people in the town figured out that those words must have been in our Czech language textbooks, and of course they were right.