I know as someone growing up as a monolingual speaker of English, I always wanted to become bilingual. I thought it was amazing that people could function within completely separate vocabularies and ways of speaking and could connect with people that I could not. It turns out, beyond connecting with people, bilingualism as large benefits for the brain, too, which can translate into strengthening bilingual people's executive functioning. This article explains it here, though for the best explanation, view the video from the article that I embedded below:

While there are clear benefits for students to be bilingual, it's critical for educators to realize that bilingualism is not something that happens automatically.

A common myth, especially in American schools, is that students that speak a non-English first language need English-only environments at school in order to maximize their learning of English. It goes along with the myth that "the best way to learn a language is to immerse yourself." The reality is that students are at a greater risk to LOSE their first language than to not learn English. Research is showing that students are losing their first language at faster rates than ever before in the United States, and this has major consequences beyond families losing connection with their families and culture (Baker, 2011). A strong first language helps students with the abstract thinking and language processing needed for academic subjects (Crawford, 2004). It also enhances the student's bilingual brain--which, as this video explains, has its own benefits in itself.

Not only that, while immersion might be effective for an adult, children are at a different stage of development. What studies of bilingual programs and translanguaging have shown, in fact, is that students actually learn languages better if they are allowed to use their first language, and a mix of the first and new language together in the process (Crawford, 2004). They are also more likely (or certain) to get behind in school if they are forced to learn new concepts in an unfamiliar language--furthering the achievement gap (Baker, 2011).

So what does this mean for our classrooms? Allow students to use their first language during class. Pair students with "language partners," or other students that share a similar linguistic repertoire, so that they are more likely to use all of their linguistic tools. Allow students to respond to content in their first language, regardless of whether or not you understand it (there are a million translation tools out there, teacher friends). Provide word walls in your room with both English words and definitions and the translated versions, so that students can grow both languages. Bring students' home languages into discussions about grammar. Have students create multilingual projects to post around the classroom. In short, celebrate and utilize ALL the language skills your students have in your classroom, and everyone will reap the benefits. 

References

Baker, C. (2011). Foundations of bilingual education and Bilingualism (5th Ed.). Clevedon: Multilingual Matters Ltd.

Crawford, J. (2004). Educating English learners. Language diversity in the Classroom (5th edition). Los Angeles, CA: Bilingual Educational Services, Inc.