This post is about people who learned the language as adults, and generally function fine in English in most workplace and social situations. But I have learned:
1. A lot of the time they can't hear shit.
English skills that are perfectly good most of the time can vanish when there's background noise, cross-talk, mumbling, or any other degradation, which a native speaker can intuitively filter out thanks to the redundancy in the signal. It's very similar to having hearing loss.
2. Sometimes they are fake laughing
If they're with a group of us, it's easy to miss the point of a joke that everyone is laughing at - either because of cultural differences, or because they missed a critical word. Maybe when they set out they only wanted to laugh honestly, but after a while many will go ahead and laugh along, to not be the odd one out.
3. If there are more than three of us, we forget they exist.
Even if we know they speak ok English, there's still that little bit of friction of not being sure if things will be understood the first time, and if there will have to be some awkward conversational repair. For most of us, that friction is enough to unconsciously drive us to interact more with everyone else. After a while we even are not making much eye contact.
4. Often, they don't know what the fuck is going on.
Lacking the ability to fully engage with the rapid-fire discussions and negotiations that go into group decisions, they often don't have an influence, and many times don't even know what the group is going to be doing next. If you can, it's nice to see if they have any needs or concerns to make part of the decision making, even though it takes extra time. But a minimal thing you can do is to keep them updated about what the group is doing.
5. Sometimes they just tune out.
If a person who learned English as an adult is staring into space while other people are talking, there's a good chance their language ability has checked out, at least for a moment. Being distracted or tired can shut off even very competent language users, and speaking English for hours can be exhausting, especially under challenging conditions such as background noise, so there may be a "Cinderella pumpkin" moment where their English just leaves for the night.
6. Jokes about their English mistakes are never as funny to them as they are to us.
It's very simple: jokes are less funny when they have to be explained.
7. Sometimes they want to have their English corrected, and sometimes not.
This varies a lot by personality and by situation. It's just important to remember that when you correct someone, for that moment you are placing yourself above them: you are the teacher and they are the student. And it can derail the conversation for a bit, especially if they are working on communicating something earnest. It's probably best to start off not explicitly teaching people but just focusing on communicating until you get a feel for what that person wants as far as teaching. (for some people repeating a sentence back the way you would say it, and then smoothly going on with what you were saying, could be helpful without being intrusive)
8. We don't know what their real personality is.
Maybe he tells long winded stories full of bragging. Maybe she's a bit of a thoughtful philosopher. Maybe he's a chatterbox gossip. Maybe she continually says terrible puns, even after everyone begs her to stop. We don't know, because these things don't come across when every sentence has to be carefully constructed. In fact, if you are not fluent there are only two personalities you can adopt that consistently work: quiet pleasant person, or highly physical extravert/comedian.
9. We are constantly sending unconscious signals of belonging.
Way beyond basic language competence, people from a shared culture have a rich body of shared references, turns of phrase, assumptions, and body language. So by definition, a newcomer to the language and culture doesn't know how to do that. Even when all the words they're saying are correct, the other signals they're sending may be off. If the person has even a touch of social anxiousness, it can make them very withdrawn. This is especially important to remember for people trying to use humour in a mixed group, such as a scientific lecture: if being immersed in your own culture is like sinking into a warm bath, being an outsider due to references can feel like an icy wind on the nipples. I just watched Deadpool, and it referenced Ferris Bueller, Rent, Bernadette Peters, and Regina, Saskatchewan, all just right and affirming for a 36 year old canadian dude like me - and all of which would be meaningless to practically everyone I met in Europe in the last two years. So the flipside of the cultural references that can connect people together is that they might be leaving folks out.
In case you haven't guessed, this is really me grumbling about my experiences trying to function in Italian. Actually it was the most wonderful gift and privilege to get to hang out in groups of Italian speakers, and everyone was super nice and helpful, even at the beginning when my speech was very primitive and a pain in the ass for all. After two years of learning and growth, though, I was painfully conscious of the decade or more it would take me to go from communicating basic stuff under ideal conditions, to being fluent and comfortable in the language and culture.
I waited to write this until I left, so people wouldn't think I'm trying to be treated differently. But I wanted to share the concrete tips, and little bit of compassion, I gained from being on the other side for once!