My dc started to talk oddly - not the accent at first but v. Odd intonation followed by accent. They talk 'normally' now out of IS environment, but they don't talk like me 
We return to the UK quite a lot with the kids ( most school holidays) and have english tv because it was important for us they didn't sound foreign, so they watch bbc and hear current language, so they keep up, and also culturally. Eg de and i will occasionally ask eg did you watch xxx (eg doctor who) when you were a kid. Oh yes, i used to hide behind the settee etc. i want our kids to also have a cultural connection to uk.
another thing i picked up on in our year in IS, while many families are 'normal', there through work and fees paid by company, many many families are wealthy to fabulously wealthy ( kids with body guards wealthy), and most kids were flying off every holiday to HK, Mauritius etc and ours were getting the dover- calais ferry.
They were exposed to a very privileged environment and we had to work hard to keep their feet on the ground and avoid becoming materialistic or comparing.
I think the cases where it works, is when they do keep moving every couple of years, so it's normal, and stay in the international bubble..
The people who say so many moves made them outgoing and find it easy to mingle and settle in etc ; we found that with ds1, but it badly affected ds2 and 3, and both are now very reserved and find settling into new situations extremely difficult. they really really need to stability and continuity. I think it would have really damaged them to keep moving. I think it suits some people but some personality types can't handle this change easily, so it's worth keeping an eye on this.