I happen to coincidentally have an insight into this very thing. When I was seven years old my father married a woman from Bogota Columbia, South America. They were only married for about 4 years. BUT during that 4 years I became utterly fluent in Spanish, due to the fact that we had a Spanish speaking God Mother to my step mother's child who lived in the home with us. She spoke NO English and I spent a great deal of my time with her. She taught me her language and as I became fluent I attemtpted to teach her mine. She was, however, over 70 at that time, so her learning did not progress at the rate that mine did. My step-mothers family had many many gatherings and there was a little girl relative my own age, Teresa, and she and I were fast friends. The family was always asking for me to speak in their language so fluent I had become that it impressed them. BUT-I thought in English for sure. I even related the Spanish to English when I was composing it in my mind.
My father and this woman were later divorced (they had married only to secure her citizenship apparently, but he wasn't in on the secret! lol) and as the years rolled by I slowly lost that language as it was not in everyday use. I did take Spanish in High School and passed with flying colors and still recall many phrases, etc but the fluency is long since past. I do recall speaking that language though...so I hope that offers a bit of insight, considering English is MY first language...