Good point. Not knowing english well enough is a good technical barrier. I've been working on a project in english for the past two months, and although I speak and write sufficiently good and fluently, I still see a huge distance in front of me. I can't think, express and discuss so easily. It's like I've slept badly for some days. It's frustratingly. And it takes time to overcome, so any test period that you have with a candidate won't be enough for that to be overcome (not that I propose hiring people for a couple of years to see whether their english levels improve, mind you)