The language barrier is pervasive even if the consultants speak English fluently. At my last company, we hired consultants from Belarus and besides a small accent there were no problems communicating with them. However, when it came to actually coding, the differences in language were more visible.
It was mostly simple stuff. the difference between an "Order" and a "Subscription". The knowledge about western addresses (we had to explain what a postal code was). You could always tell if it was one of our consultants that wrote a given piece of code.
That being said, it was effective. We accumulated quite a bit of technical debt that we later had to pay off, but by any metric, it was successful. They were very competent programmers (one was amazing!) and I wouldn't hesitate to bring them onto another project. But I would try to micromanage more.