Bilingualism is the ability to speak two languages; multilingualism is the ability to speak more than two languages. Years ago, the common thinking was that speaking more than one language was confusing for the brain and impeded cognition. Since then, studies of the brain have confirmed that bilingualism actually improves cognition and may even be an effective tool to help with dementia. In today's workplace, bilingualism is also an advantage; no matter what the second language is, there is undoubtedly a need for workers who know another language. Two of the links below discuss the benefits of bilingualism.
Since you tagged this question in "Literature," I have included a link to a good eNotes discussion of bilingualism in literature. In short, using a second language in a written work can enhance the meaning of the selection as well as provide insight or clarity to a character.