These are completely different words.
Half-breed is an historic term used to describe anyone who is of mixed Native American (especially North American) and white European parentage. Métis is a more general French term for mixed race, which has generally referred to a person of descent from two different major ethnic groups, such as European and African, European and Native American, or European and Asian. Hence they are a mix of different ethnic groups.
Multilingualism is the act of using multiple languages, either by an individual speaker or by a community of speakers. Bilingual is a special case where a person or a community speaks two languages.
The two obviously are very different. One signifies the race and the other, the language.