Being legible also means to cater to your audience. I work in an English-speaking company in a country where English isn't the native language, with loads of non-native speakers from around the world. Sometimes the native/best English speakers are the ones being misunderstood, because they use idioms or advanced words. None of us are bad at English, and I don't mean that I need to "dumb it down" (if anything, verbally I'm one of the worser ones), but I don't feel like I'm missing out on speaking simple with an accent.
Generalizing from my own experience, it’s easier for me to understand a non-native Spanish speaker than a native Spanish speaker and I would guess that the same applies with ESL speakers. One thing I found really fascinating is that even though I’d never studied French¹, I actually had an easier time understanding a conversation between my ex-wife and her aunt in French than when they spoke Spanish in which I was functional (my skill in the language has gone up a great deal since then so that I now read fluently, and speak and listen reasonably well, albeit less well than I would like).
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1. Thanks to my kids studying French on Duolingo and my joining them, I can no longer say that I’ve never studied it.
>Generalizing from my own experience, it’s easier for me to understand a non-native Spanish speaker than a native Spanish speaker and I would guess that the same applies with ESL speakers.
You guessed right -- it's /usually/ easier to understand other non-native speakers, both because of accent and less idioms. That is unless the accent is really heavy and doesn't match your own.
I don't think it's really to do with accent, but rather that non-native speakers tend to talk slower and use simpler grammar and a more limited vocabulary (including fewer idioms).
That does help, I’m sure. There’s also a tendency to incorrectly apply foreign grammatical structures to the language in a way that might be strange or incomprehensible to a native speaker while making perfect sense to a fellow second-language speaker.
I've read of a language, or I guess dialect of English called "European english", i.e. English as spoken Europeans that learnt it as a second language. Another example is how a Brit using too many idioms like "Bob's your uncle" would confuse English speakers that haven't been exposed to Britishisms.
Searching on YouTube just gives me this relevant result, an English that has some new/odd words that's been adapted by the Euro-bureaucrats https://www.youtube.com/watch?v=vf8KxvRYvQU
