Now I find evidence that could support this theory, however self-explanatory it already is. In the Facebook Polyglots group, a Finnish man writes:
"I have wanted to raise my children to be polyglots. But, I have found it challenging to teach them two languages. I've spoken English to all of my three sons from day one. They all speak English, but have trouble remembering basic words and make beginner mistakes. Just today, when I called my eldest son and asked: 'Are you home already? Is your mother there?' My 13-year-old, with whom I've spoken English every day of his life, says: 'He's home. I think he's upstairs.' With unconcealed frustration, I said: 'She, not he.' 'Oh! Sorry, I forgot'."
What a pleasure in finding other people making the same mistake! Seriously, this gentleman's children can speak a little of multiple Romance languages, attesting to their language capabilities. And yet a mistake is made because "he" and "she" are the same word in Finnish, hän, not just the same in pronunciation, but also in spelling, a stronger case than in Chinese we may say. In fact, the same can happen in a few other languages. I remember a polyglot friend of mine told me his Armenian friends sometimes make this mistake too. In Armenian, again, one word, նա, can mean both "he" and "she".
To permanently solve the problem, you have to think in the language you speak, English in this case. If the thinking process is in Chinese, Finnish or Armenian, and speaking is after a translation, possibly a very fast or nearly subconscious one, the risk of making this mistake still exists. But before achieving the level of thinking in English, the best the student can do is speak slowly.