My experience is of a parent of a child whose first language is not the school language, and as a language teacher myself.
As Hag pointed out, it does make a difference when the L1 (first language) of the EAL children are different vs mixed. When they all speak the same language they will tend to play and form relationships with each other in that language and it becomes a little bit insular. Certainly DS picked the one other child who spoke English and stuck to him like glue (they are in different classes now.)
Additionally language teaching/support is very different when you have one L1 vs a range of L1. Where there is a range, you're limited to teaching English in English, and basically they pick it up from immersion, especially in a school setting where there are unlikely to be actual ESOL classes.
I think it's likely to be a very positive thing actually. IME children of this age learn very fast - DS picked up basic German in 6 months and close to fluency in a year. The pace in Reception should not be too fast - there should be a lot of repetition etc, this is helpful even for the native speakers because it means the pressure is off. It's still very play based.
I would imagine if your DC has any additional needs, they might be disadvantaged because the language issue itself is an additional need and might take up time, but if they are NT and fairly bright then it shouldn't be an issue.
And for playdates etc - as a mum who doesn't speak the native language here, please please ask anyway! Usually they want to ask you, but they're too embarrassed, and probably worried about inviting over a child they can't talk to. It's not too difficult to understand if you keep it simple (Can X come to our house, to play? With actions, or add "you come too, for coffee?" with actions if they look nervous.) Swap mobile numbers - SMS is pretty much universal for text and it's much easier for them to decipher a text message (you can even translate them with an app) than feel embarrassed and flustered on the phone.
Somehow it's easier to set up playdates after a party or something, but do try - they're probably feeling even more nervous than you are, and it's much easier to respond to (basic) conversation as a non native speaker than to start one, I'm always worried that if I open my mouth and speak German to somebody that they will rabbit on for ages and I won't know what they're saying, but if they ask me a question then I do try to answer.