You and your family — which includes at least one school-aged child — have just moved to a new country with a whole new language, or are about to take the leap. You've got several big decisions ahead of you. Your child's got to be educated, obviously, and they also need to learn a new language to begin their process of integrating into their new society and all that involves. Hopefully, they'll make some nice friends and begin feeling at home in their new surroundings, too.
Some parents are going to try to catch several birds with one stone and choose the local public school, where they hope their son or daughter will pick up the new language, learn about math, science, geography, history, and all that other stuff, and develop a good social network.
Can Your Child's Potential New School Really Help Your Child Learn A New Language?
The process by which a child acquires (a word that itself represents "absorption" rather than "learning") their native language or languages is fascinating, miraculous, and frankly still poorly understood. I've just spent more time than I'd like to admit reading about how infants begin acquiring language, and despite the fact that quite a few of those things appeared in peer-reviewed journals, people regularly come to quite contradictory conclusions. A few things are indeed clear, however :
- Different children don't necessarily use the same internal mental strategies to master new steps in the linguistic process, as evidenced by the fact that some use single words, while others go straight to chunks.
- Language acquisition is hard work that doesn't go as fast as we might think — the process starts as early as the womb, though the first spoken words obviously come much later.
- Language acquisition requires imitation and context. As the paper I'm here referring to says: "No one—adult or child— has ever learned a language by listening to the radio." Though the exact way in which children attach meaning to words isn't clear, it does have something to do with the environment and human interactions (which, unlike the radio, are a two-way street!). It's not always as simple as pointing to things and labeling them, though that works quite well for nouns, but it does necessarily involve context of some sort.
When your child was busy acquiring their first language(s), you and other significant people provided this context, and this opportunity for imitation. Good language immersion schools (which may be dual or single language) mimic this process. Having read more than a few web pages in which immersion schools explain their philosophies, I know they can differ quite a bit, but also that they all have a philosophy — one that mimics the natural process by which children acquire their first language in infancy and beyond.
Schools in areas with many immigrant students will likewise have adapted to this reality somewhat. They may have teachers who speak the native languages of at least some of the children available for extra support, and they may have teachers especially trained in helping students learn the local language. Students will have lessons specifically to help them become more proficient at understanding, speaking, reading, and writing their new language, and teachers are often cognizant of potential cultural challenges as well. 
At the very least, teachers will be familiar with basic things that can help and hinder language learners, things such as :
- Pronouncing words more slowly and clearly than one would with a proficient speaker can aid the learning process.
- Language learners are going to have trouble with idioms and euphemisms, so it's helpful if teachers can avoid using them.
- Repeating what the learner has said in a more grammatically-correct form can help the student develop better syntax.
- Visual aids are your friend.
(That is where language lessons, time with new friends and others who are indeed able to provide context , subtitled films , and the like come in. Sending your child to a school that isn't equipped to help them learn the new language doesn't mean they won't learn the language. It probably does mean your child won't learn much else until they do become proficient, though, since all the other subjects are offered in a language your child doesn't yet speak.)
Are Children Really Better And Faster At Learning New Languages?
Not necessarily, no, and learning a new language as a school-aged child isn't the same as growing up bilingual either.
Some research confirming what most of us already suspect — that some folks are simply "wired" to learn languages more easily — does exist . It is, of course, possible that your child or children fall into this category, in which case they'll likely have an easier time adapting to their new society and new school.
I've been quite surprised to observe some of the attitudes parents of immigrant (typically white European, including British, in the unscientific sample I got to interacted with) children have, though. "Don't worry, it'll come", is the overwhelming sentiment I came across. It might not — and your child may continue to struggle.
Barry McLaughlin's paper Myths and Misconceptions about Second Language Learning: What Every Teacher Needs to Unlearn  offers some insights that may shock or awaken the parents of young language learners who assume they'll have no problems in a new school. There's really too much material there to be able to share it all here, which itself heavily refers to the pre-existing body of research, so I'd highly recommend you give it a read for yourself. Some of the highlights, however, include:
- Many children absolutely won't be better language learners than adults and adolescents. It may seem that way because children aren't expected to have as broad a vocabulary or as varied a grammatical playbook as adults to function in their new language, but becoming proficient is hard work for children.
- More exposure doesn't necessarily mean faster learning! Research shows that kids in full immersion programs do not do worse than those in intensive language-learning classes. (What this means for you, as the parent of a language learner in the process of making decisions about their education, is that you should know that your child's language progress isn't hindered if they learn other subjects in their native language. They'll certainly learn more about those other subjects if they study them in a language they actually understand well.)
- Conversational skills don't necessarily mean total fluency. A child who is skillful in a conversation may really struggle with the written word, and without teachers always being aware of this problem.
- One of the most important lines from the paper is: "Second language learning by school-aged children takes longer, is harder, and involves a great deal more than most teachers have been led to believe."
'Why Are You Being Such A Debbie Downer?'
I do think it's important to get the fact that language learning typically isn't easy or hurdle-free across, however. Trying to learn a new language and all the other stuff kids learn in schools is a momentous task. I'd recommend one of two approaches:
- Accepting that your child will learn the new language but nothing much else during that first school year in their new country, if they're attending a local public school without a specialized approach to language learning. (This may mean repeating the grade the following year to learn all the other things.)
- Focusing on intensive language-learning classes while ensuring that all the other instruction takes place in a language your child does understand, whether by homeschooling or making use of distance education, or enrolling your child in an international school where their native tongue is spoken, if possible.
In addition, these things will help:
- Regardless of your school choice, provide plenty of intimate access to native speakers.
- Keep track of your child's progress, discuss their challenges with them and their teacher(s), and step in to find additional resources where needed — whether that's to advance conversational skills, help your child improve their spelling or phonemic awareness (not applicable to all languages), work on some grammar, or get better at the more difficult aspects of pronunciation.
- Be patient, praise your child for progress, and don't make them feel bad for experiencing challenges. ("Duh!" — I know.)
In addition, support your child through the difficult transitional period and provide cultural grounding as well as encouragement in the assimilation process. The transition after moving schools is already difficult enough in itself, but your child is dealing with numerous other changes too.