What language do the multilingual THINK in?

There’s some info you should put before the real question. It’ll be quick, don’t worry :D

1. How many languages do you know (Must be FLUENT)
2. What are they (Optional)
3. What were your first language(s)?
4. How many fingers am I holding up?

Ok, now that that’s over with, to the real question. This came to me in Spanish class, as my teacher is bilingual.

What language do the multilingual THINK in? I would guess maybe their first language, but I don’‘t really know, so I could use the collectives help! Thanks a lot!