Speaking more than one language (or multilingualism) is not a direct cause of a stammer. Stammering affects people who speak all languages – including those who speak one language as well as those who speak two or more languages.
There is some research to suggest that if a child is less proficient in a language, they may stammer more as they process the language. For example, when children are learning more than one language they may have higher rates of speech dysfluency, which can sometimes be misidentified as stammering. There may also be different cultural expectations of language use that can affect the stammer.
Whichever language a person speaks, the aim should be to help them speak fluently and confidently in all their languages. If you are concerned about your child’s speech and language development, then you can always talk to your GP or speech and language therapy service.