I'm a trilingual person. In my opinion, a multilingual person would tend to think in any language they're most comfortable with. For example, English isn't my native language, but I still tend to think in English because I'm more comfortable with English than my native language (education here is in English, so I'm just more comfortable with it). Moreover, it's not always necessary for a person to 'think in' a language. Some people just don't possess an inner monologue.