Hi there! I have started a new relationship and my girlfriend has a bilingual son who speaks French and English. She has asked me to teach him Russian... Should I? Will it not interfere with his other languages?
If he is bilingual then french and english will both be his mother tongues, so a new language wouldn't affect them. Very young children don't always separate their languages at first but learn to slowly, but if he is already speaking both french and english well he has probably passed this stage. Go for it!