I can kind of relate. My first language was Spanish, and I grew up in a Spanish speaking country, but I went to a bilingual school that also taught English. I guess when I was a kid I thought "Hey! I stutter less in English" so I began to default to English. I stutter so much in Spanish and my English is so good that people assume that I don't know Spanish that well. I've been stuttering less lately, but it still happens.