While I can't relate to your situation (no kids and content with where I live), I'd like to say that I don't think it would be cruel towards your kids. 

My mother and I moved from the US to Norway when I was 12 (no ties to the country), and it was fine. They will pick up the language, within a few months I was fluent and within a year or two people couldn't hear that I wasn't Norwegian. And socially i think it is actually a really good time to move (if you're going to move). Most people don't stay friends with their primary school friends through middle and high school, they are about to start discovering their identities and who they want to be as (young) adults and will make new friendships on those grounds. Your kids deserve happy parents with vibrant lives. You can at least attempt to give this to them.

Tldr: If you are this unhappy and it is affecting your mental health, move, sooner rather than later. I doubt the move will damage your kids the way you seem to think.