I actually had a small headcanon that Marinette wasn't fluent in Chinese or knew very little so when I saw the new episode I had a moment of surprise. Though at the same time I also have had experience myself. My parents are from the Philippines and when I grew up only English was spoken in my home. I'm guessing it was the same with Marinette as since she was living in France, French would be the most spoken language at home. I'm still surprised!