What language do people think in?

People think in any language which they know or not in any specific language. I was tri-lingual from early childhood and have learned other languages since. I am also a mathematician by profession.

For non-mathematical matters I think in any one of my first three languages. If the language that I am thinking in does not have a word with the nuances or subtlety that I am looking for, then I switch language - on the fly. I stick with that language until the next nuance blockage. For example, English has a poor vocabulary for describing flavours (other than by comparison).

Much of my mathematical thinking is abstract or thinking in numbers or shapes. This may seem bizarre to a non-mathematician but I recently discovered that my physicist daughter does likewise. Also, most of my early mathematical education was in Gujarati so I think my times tables in Gujarati.

Generally they think in their native language. When learning a new language a person will think in their native language, then the new language, and back to the native language. As they learn the 2 language they will stop thinking in the 1 st language and go directly to the L2.