That is a really interesting question ItsAColdDay, and the answer isn't simple, there are a lot of factors to consider.
for example, is this country the home country of one of the parents, and therefore the language is the mother tongue of one half of the family? If so, even if the language isn't spoken at home, but the family visit their home country every year, there is a high chance that they will retain it. (obviously if one parent speaks the language then it would be ideal for them to speak to the kids in that language)
If the family continue to have links and contact with the country, and possibly persue contacts with it, then they may retain the language, I know one family who returned to Australia. The kids were fluent in local language and had been to local school and had local friends. Eldest was about 14, he never lost it, twins were about 12, they also kept it, particularly the girl who kept in contact with her best friend.
The youngest was about 10. She really missed speaking it, so her mum found a Russian speaking art school that met on a Saturday, and the youngest loved it, loved speaking Russian there, and so kept it.
Another family, kids aged about 8, 7, 5, returned to home country due to medical emergencey. They had lived in a village, and spoke the national language and the local language both fluently. They all lost it completely. Parents tried to keep it up at home, but the kids weren't having it, they wanted to fit in and didn't want to speak this 'other' language.
But both oldest then learnt the language as their second language at High school/university, and guess what? Although they didn't consciously 'remember' it, they became fluent again, with faultless accents and perfect grammar, pretty much native speaker, which is rare for an adult learner.
My ds and dd were tril-lingual aged 6 and 4. They only speak English now, although they understand quite a lot of one language we still have contact with.
BUT, and it is a big but, being bilingual is great for the brain, it develops all sorts of areas we don't fully understand. Learning another langugae later is much easier if you have learnt one as a child, even if it has been forgotton. My kids are really confident in non English settings, they are totally unphased by being in a situation where they don't speak the language etc. It helps understand other peoples worldview and makes them more citizens of the world. It is a very positive thing.
You can never predict if you will stay permanently in a place, but if the plan is to stay for life then local schools are the only real option, otherwise your kids end up stateless, not belonging to either place. If the plan is long term, eg 10 years, then I would usually recommend that you do at least kindergarten in the local school, probably 3-4 years of primary too, then switch to English, and do Secondary in English.
Even a short placement (2 years) for primary, I would seriously think about doing some of it in local school to get the language, most kids can happily take a year out and do a year in another language.
I personally would do this even if the local system wasn't brilliant, if the local system is not great then I would limit it to 2 years max in local school, and do English lessons alongside to keep their English education up.
The bottom line is, a family will only be happy in a place if they integrate well, and kids with no language don't integrate, except to the ex=pat community