Unless you have dual nationality you'll probably find its quite difficult, if not impossible to live for a year in a foreign country without having a work visa sponsored by a local employer. If that's not an issue for you, I'd definitely recommend Sweden, which has an amazing lifestyle, and you can get by to an extent without learning the language.
(From the mention of "sabbatical" I'm guessing the OP's wife is an academic, which will make this straightforward IME.)
Sweden's great, but in your place I'd rather think about Finland. Helsinki is one of my all-time-favourite cities. Finland's school system is legendary - and famously inclusive, so I don't think you're going to find a self-contained gifted classroom, but you can expect to find really good teachers. Apart from anything else I'd love to hear a first-hand report of what Finnish schools *are* like for HG+ children! Fascinating and very beautiful language (non-Indo-European), but as K&K said, practically everyone speaks good English too (so much so that as adults, you'd have to make serious effort if you wanted to speak Finnish, but your DS would learn it at school I expect). Easy access to Tallinn (we travelled there from H. by ferry, and it's amazing - 14th century city walls etc.), St Petersburg, etc.
If you're going to all the trouble of uprooting everyone, switching schools etc., it would seem a bit odd to me to do it just to go somewhere else in the same country! But now someone will tell me that I just have no idea how diverse the US is, and they're probably right ;-)
