Well, this is very dependent on exactly what you mean by ‘fairly good English’. I’d say that getting to this level is hardly trivial — I’ve seen many non-native speakers whose writing is nearly unreadable. Though then again, there are a fair number of native English speakers who also have terrible writing. As a monolingual speaker I can’t personally compare this aspect of English to other languages.
I'm thinking of things like .. in Slovak, spoon vs teaspoon is lyžiča vs lyžička. If you don't understand how they mutate the word endings - and as far as I can tell, memorise them all on a case-by-case basis, this is easily lost. small spoon, little spoon, tea spoon, there's not many ways in English for it not to be understood.
So my understanding is not so much that it's easy to learn, but the MVP of being able to communicate is a surprisingly small subset of the full language.
Isn't that just the diminutive? The English equivalent would be a "spoony." You might get looked at foony if you use that word to ask for a teaspoon, but if an English speaker understands what you mean, it's because they know their language well enough to guess the meaning based on a handful of words and the context.
Similarly, I'd expect a Slovak speaker to understand "lyžiča malé", "malinká lyžiča" and "čaju lyžiča", even though they probably sound horribly wrong. (BTW, my dictionary says it's spelled "lyžica".)
In Polish, a screw is "śrubka", and "śruba" is a big screw - also a ship's propeller. "Kowadło" is anvil, and "kowadełko" (small anvil) is the anvil bone in your ear. "Łyżka" is a tablespoon and "łyżeczka" is a teaspoon, not merely "smaller spoon". "Suka" is bitch (either pejorative or very technical term for female dog), "suczka" (little bitch) is what dog owners and breeders actually call female dogs. And so on. Diminutives of words can end up having quite different meanings to the original words.
It's all true, and I'm not pretending English is a trivial language. My point is, you can speak fairly good English with no knowledge of any of these complications. You simply cannot escape from fundamental complexities in all the European languages I know, that need to be navigated for even simple communication. It helps that the tenses are at least somewhat organised. This "Cartesian" model is much clearer to me than e.g. the French "bag of tenses".
Of course you can argue what's "basic communication". Something like "I go train Manchester tomorrow" is fairly unambigous, but very poor use of language. My impression and point is that it's much easier to speak simple, correct English than it is to speak simple, correct {French,German,Polish}.