> Can sliding windows be properly insulated?

Yes. Beyond that, if they didn't work they wouldn't be used. Continental climates get much colder than pretty much anywhere in Europe, outside of a select few areas.

> In general cold is a bigger deal in Europe than warmth, and will continue to be so.

Masonry is a bad match for cold. The structure acts as a high velocity heat conduit and the earth eats all the heat you produce. Europe's winters (in general) are extremely mild, arguably even more so than its summers.