There have been very few studies on bilingualism and multilingualism and how they affect dreams. These are small studies, but they certainly find that people who speak any second language, even without good proficiency, at least occasionally dream in the second language.
One study asked the subjects what they thought made the difference, and they said that it was determined by the people and or the setting that was being dreamed about.
If you thought of your family back in your country of origin, it’d likely be in that language regardless of whether it was now your dominant language. And if you were dreaming about people you’ve known as a young adult, living in another setting where you spoke a different language, you’d be dreaming in that language.
