I guess what she means is "I am Thai, I know when to use it"
In every language, depending with whom you're speaking with, you won't use the same words. I don't use the same words when I speak to my friends, my colleagues, my customers or my boss. But I know the difference and won't call my customer a cheap charlie (at least not in front of him) because he chose the basic options.
By using the formal language you have less chance to make an embarrassing mistake, I guess it's what she means