I've actually had a harder time conversing with people in some parts of Scotland by phone, due to our different accents, than in some parts of Europe. Even though we both speak English as a first language. The UK can have bigger internal differences than the rest of the English speaking world.
Some of this is down to the international (US) English used in non-native English speaking places. I had to write my book on ASP.NET Core in American English, which took some getting used to. :)
You also make moving somewhere more complicated. When I moved from UK to Paris, I was fine with work, with socialising, but my French was (and still isn't) nearly good enough to deal with government paperwork, appartment searching, reading up on tax law... etc. Sure, this problem can be solved either by the country making it easier for forigners, or by people moving paying for somebody to help them in these areas, but again its another barrier and/or additional cost.
There's also the matter of local work-related communications. Anything from taxis to tax offices, courier/mail services, etc...
Second, you may need a decent command of the local language to access services and deal with the local authorities. For example, getting your car back if it's towed. Figuring out whether you can get a visa for that foreign consultant. Expressing your views to your political representatives. Getting emergency care at a hospital. Understanding laws written in the local language. Passing a driving test. Talking to your children's teachers. Working out how much you'll be taxed on that thing you're considering having imported.
The problem is that if you want to attract non-local talent, it's harder to attract them somewhere where they'll have trouble with day-to-day life. English-speaking countries have a big advantage here: no matter what nationality a professional has they probably speak English well enough to be comfortable living there.
As another poster mentioned there are cities where the overall English proficiency is so high that it matters less (Stockholm, Berlin, Amsterdam, ...)