It's not easy to become highly proficient in english but it's quite easy to speak just barely well enough to communicate effectively in a professional context. Importantly, the written form follows naturally from the spoken. You won't get all the edge cases right (that's incredibly difficult even for native speakers) but getting in the ballpark can be done purely phonetically with a fairly small set of rules. Combine with modern spellcheck and I expect it's pretty difficult to beat for ease of practical use.

I think at least a few of the latin based languages are in the same ballpark but for inane historical reasons it's english that won out.

Compare with chinese where even if you sweep tones under the rug you've got a bunch of idioms (difficult) followed by one of the most difficult writing systems in existence. Don't get me wrong, I think the writing system is quite elegant and has a truly impressive history, but neither of those things has anything to do with ease of mastery.

A tangential thought is that if you intentionally set out to come up with a rule following yet maximally difficult language I think a reasonable approach would be to fuse the equivalent of latin grammar with chinese tones and then fuse a chinese style writing system with arabic style contextually sensitive ligatures.

> Russian is another “difficult” language, but all the satellite nations had no problem picking it up.

Russian is not more difficult than English and a lot of the satellite states were speaking other Slavic languages. If you already speak Spanish, it's less difficult to pick up Italian too.

There’s also the fact that a huge portion of foreign immigrants to the US don’t and won’t learn English, but can still operate just fine (or even have the system cater to them - press 1 for Spanish).

Look at the uproar over requiring commercial drivers to be able to read road signs in English.