As immigrants from around the world enter the United States, schools have long focused on teaching them English. But bilingualism is gaining favor among employers, educators and parents, fueling a movement to help children who are native speakers of Spanish, Mandarin, Arabic and other tongues master those first languages. Educators say many first-, second- or even third-generation immigrant children speak socially in what is often called a heritage language but would be lost if they had to write an essay or a formal letter in that language. Research has shown that proficiency in a native language can help students become better English speakers. In an increasingly global economy, it also makes business sense.