This is just anecdotal, of course, but I've found that developers from Eastern European countries tend to speak English that is more than good enough even for c

This is just anecdotal, of course, but I've found that developers from Eastern European countries tend to speak English that is more than good enough even for complex ideas. Accent can be a slight issue at times, but not fundamental language understanding. Time zone is definitely an issue though. Especially if you starting using multiple teams in many different time zones. It's one thing if you have Team A in, say, San Jose, and Team B in, say, Ukraine. But when you take that and extend it to include a team in China, a team in Japan, a team in North Carolina, and so on, things start to get really tricky. I think 2 time-zones is fairly manageable. More than that is, in my experience, pretty damn challenging.