That’s likely because the languages you speak all share recent ancestry (if they’re not all Romance languages, they’re at least all descended from Proto-Indo-European.)

Meanwhile, I’ve seen e.g. Mexican and Chinese ESL speakers have very bad mutual intelligibility, because they make very different mistakes.

That's true. Assume a monolingual learner. You can predict the kinds of mistakes they will make based on their native language (phonetical, phonological, semantic, etc.). As a person learns more languages, they will likely get better at decoding these things. Particularly cultural based assumptions used in languages.

But as an ESL person myself I’ve never thought of English as a language about rules as much as memorizations. It’s the x86 of human languages, for better or worse, and trying to systemize it fully was how I messed up English most as a kid and how my parents tried to teach it to me.

Another funny one I ran into a few times was how for a time like 8:30, we'd say "half eight" in the UK, but when I would casually say this in the US people would be like "half eight? you mean four?" It had never occurred to me this common British parlance would be unintelligible elsewhere. Of course, there are just as many things the other way around, although watching a lot of US TV will take care of that.

In dutch, half eight is 7:30 too.

7:40 is weird: "ten over half eight"

That said, the way the French count numbers is, I think, universally confusing. Knowing the French, this is intentional :P

So for instance if I might say “These three things,” he would say “these tree things.”

He was clearly able to make the sounds but in some cases he couldn’t. I never felt comfortable asking about it because I thought it might be a class thing.

The other one is asking for a cheese sandwich - that seems to throw Americans every time.

"Wow, you mean you can have a conversation with a French person?"

"No... but I can talk to anyone else who's taken two weeks of French."

Fluent Northern European English (Belgium, Netherlands, Scandinavia, North Germany) almost blends into "normal" English to me. It is kind of shocking.

which is why the quotes were there to let pedantic people know I’m aware there probably is a better term

Actually, the word "international" is a good example -- in my regular American accent it would be pronounced like "innernational", whereas I would distinguish the T when conversing with non-native speakers.

TL;DR, it's not actually "innernational", but rather an alveolar flap on the t that you have a hard time distinguishing between depending on where you grew up.

I moved to Scotland for a year, and London for a few years later. I’d regularly here phrasing I’d never heard before and would need the speaker to clarify or would look up later. In the vast majority of those cases it was most often an old word that appears to have fallen out of fashionable usage. “Ken” being a synonym for “know” in Scotland being the one I remember most given how frequently it was used.

So it’s not just non-native speakers that struggle at times!

https://en.wikipedia.org/wiki/Tam_o%27_Shanter_(poem)

https://www.youtube.com/watch?v=IAyRoPcuykY

Comes a point where you get so used to what is, admittedly, such a weird and non sensical pronunciation, that the same mistakes I used to make as an Italian native I don't understand at all from a Spanish native that doesn't pronounce as well as I'm used to. Only after I realise what the word is I understand where they are coming from.

Also the every day vocabulary is not that big and fancy, I tend to use some "fancy" words sometimes just because that's the word I would have used in Italian, and it's correct in English, but not widely known. Then again I'm now reading Stephen king and having to use the dictionary every few pages, just because something is mostly obvious by the context but I want to double check.

By analogy: elements, valance bonds, and van der Waals forces in chemistry. All matter, some are stronger, subtle variances can have profound differences.

Regardless, it is very frustrating to know the meaning and the spelling of an English word but not knowing how to pronounce it. Or the inverse, hearing a word I know but not recognising it until someone spells it for me or I see it written.

I learnt most of my English in written form so this happened to me a lot when I moved to London for a couple of years. I remember pronouncing “recipe” like “ree-syp” and my boss had no clue what I meant.

With Spanish and other languages you can learn the pronunciation rules and you’ll be able to read a text correctly even if you don’t know the words. With English you have basically to learn every word, as double o doesn’t sound the same in “door” as in “soon”. I remember being dumbfounded by the pronunciation of words like “tomb” or “womb” or “colonel”.

My modest proposal is this: it used to annoy the brits that european english differs[2] to their dialect. On the other side, we should incite the possibility to embrace[3] our dialect for international use. Finally, this would paradoxically be a gift to EN-GB speakers, because EN-US speakers might then experience being separated by a common language.

[1] http://www.europanto.be/cabillot1.html

[2] https://www.eca.europa.eu/Other%20publications/EN_TERMINOLOG...

[3] as in s'pore, lah?

My house has folks from France, Burundi, US and India. The non-US people find it a lot easier to understand each other's accents than Americans do understanding us.

Ironically though, I have a bad habit of using big words in spoken English, because I learnt it as written bookish English first.

As a Chilean I understand very well what you mean ;)

As you allude to, that's typical for any speakers of any foreign language, in my experience. I speak several languages in addition to English, and native speakers of those languages are often the hardest to understand, because they're the most fluent in the language.

The hard part is that most of said US/UK native speakers seem to fail to comprehend this simple wisdom of yours.