Here is a great article about the benefits of foreign speaking nannies for families considering an au pair.

Yana Paskova for The New York Times

Yashmin Fernandes, right, hired a bilingual nanny, Elena Alarcón, center, to care for her 1-year-old daughter, Calliope Castro.

Published: August 18, 2010

When Maureen Mazumder enrolled her daughter, Sabrina, in a Spanish sing-along class a year ago, she hoped it would be the first step in helping her learn a second language. But the class did not seem to do the trick, so Ms. Mazumder decided to hire a baby sitter — one who would not only care for her daughter, but also speak to her exclusively in Spanish.

Fred R. Conrad/The New York Times

Elas Tarazona teaches Nir Liberbolm’s son WIlliam both English and Spanish.

“It was a must that she speak Spanish,” said Ms. Mazumder, who said neither she nor her husband were fluent in the language. “We feel so strongly that our daughter hear another language.”

Ms. Mazumder, whose daughter is nearly 3, has company. Although a majority of parents seeking caretakers for their children still seek ones who will speak to their children in English, popular parenting blogs and Web sites indicate that a noticeable number of New York City parents are looking for baby sitters and nannies to help their children learn a second language — one they may not speak themselves.

That has certainly helped Elena Alarcón, a nanny born in Mexico who attended school in the United States. Ms. Alarcón recently completed 15 interviews with parents living in Brooklyn: Every one of them insisted that if hired, she speak only Spanish with their children.

“I thought I would have to speak English with the families,” Ms. Alarcón said. “I was surprised, they wanted me to speak only in Spanish.” Ms. Alarcón now works for Yashmin Fernandes, who became fluent living and working in Latin America. Ms. Fernandes speaks in Spanish with her daughter; her husband, who is of Puerto Rican heritage, speaks in English. “His family is the Spanish-speaking side, but I was more adamant about getting a Spanish-speaking nanny,” Ms. Fernandes said.

Parents cite different reasons for hiring baby sitters and nannies to speak a second language with their children. Some struggled to pick up foreign languages and want to make life easier for their children. Some believe it makes them smarter. And naturally, this being the melting pot that is New York, many parents have a connection to another language and want to reinforce it.

Simona D’Souza, 38, grew up in Kuwait and Canada. Even though her parents spoke Konkanese, the language of Goa, to each other, they insisted that their children speak only English. “They didn’t realize it would be beneficial to us to learn another language,” she said.

Indeed, not long ago, many parents insisted that their foreign-language speaking nannies refrain from using their native tongue and speak only English with their children, for fear another language might muddle their English-language development.

Ms. D’Souza has taken a different tack with her own three children. Her husband is German and speaks to the children exclusively in German. Her nanny of five years spoke only in Spanish with the children. “We would not have hired her if it wasn’t for the Spanish,” she said. Now, she is contemplating putting the children in a French immersion program.

“Once you are trilingual, your brain can break down new languages that make it so much easier to learn your fourth, fifth and sixth languages,” she said.

In recent years, a number of neuroscientists and psychologists have tried to untangle the impact of bilingualism on brain development. “It doesn’t make kids smarter,” said Ellen Bialystok, a professor of psychology at York University and the author of “Bilingualism in Development: Language, Literacy, and Cognition.”

“There are documented cognitive developments,” she said, “but whatever smarter means, it isn’t true.”

Ms. Bialystok’s research shows that bilingual children tend to have smaller vocabularies in English than their monolingual counterparts, but that the limited vocabulary tended to be words used at home (spatula and squash) rather than words used at school (astronaut, rectangle). And the measurement of vocabulary is always in one language: a bilingual child’s collective vocabulary — i.e. from both languages — will probably be larger.

“Bilingualism carries a cost, and the cost is rapid access to words,” said Ms. Bialystock. In other words, children have to work harder to access the right word in the right language, which can slow them down — by milliseconds, but slower nonetheless. At the same time, bilingual children do better at complex tasks like isolating information presented in confusing ways. In one test researchers frequently use, words like “red” and “green” flash across a screen, but the words actually appear in purple and yellow. Bilingual children are faster at identifying what color the word is written in, a fact researchers attribute to a more developed prefrontal cortex (the part of the brain responsible for executive decision-making, like which language to use with certain people).

Yana Paskova for The New York Times

Some of Calliope’s Spanish books.

Ms. D’Souza says that both her sons lagged their peers by almost a year when it came to verbal development. Her pediatrician recommended speech therapy, and one son’s preschool teacher expressed concern that he did not know the alphabet. But when both started speaking, at around 3 years old, they were able to move fluidly among three languages. She said that her older son tested in the 99th percentile for the city’s gifted and talented program.

“The flexibility of their thinking helps them in nonlinguistic abilities like science and math,” she said. “But at the same time the normal things — the alphabet — they have trouble with that.”

One arena in which being bilingual does not seem to help is the uber-aggressive kindergarten admission process.

“It doesn’t give you a leg up on the admissions process,” said Victoria Goldman, author of the sixth edition of “The Manhattan Family Guide to Private Schools.” It is one piece of the bigger puzzle, which includes tests scores, interviews and the ability of a child to follow directions. “Speaking another language is indicative that you are verbal,” she said, “but you have to be behaved.”

George P. Davison, head of school at Grace Church School, a competitive downtown school, said that bilingualism tended to suppress verbal and reading comprehension test scores by 20 to 30 percent for bilingual children younger than 12. “If anything, it can have a negative effect on admissions,” he said.

Ms. Bialystock concurs that for a child to retain a language, a nanny probably will not do the trick. “It’s an interesting solution; it gives young children a consistent exposure,” she said. “But how long will the nanny be around, and who else will the child use that language with?”

Some parents have taken that into account. Nir Liberboim and his partner hired a Peruvian nanny to speak only Spanish with their son William, who is now 1 and a half. Mr. Liberboim grew up in Texas and struggled to become fluent in Spanish because he was taking only a few classes a week. Knowing how hard it is to learn languages at older ages, he wanted to help his son early on, an opinion his partner shares. “We view it as a gift we are giving him,” Mr. Liberboim said.

They have decided to keep the nanny, if she is willing to stay, even after William has started school. “There’s a financial implication to that,” he said, “but we don’t want him to lose it.”