I call it the language comfort pit. Because it's comfortable to not learn the language properly and hang out with people from the home country, you just make a small home country in the new country.
It happens whenever you have that community available. If it's just yourself, then you are forced to adjust. I saw it happening with my family members too. The ones that went to the large immigrant community led to them not knowing English well, while the more isolated ones integrated properly. One even first went to somewhere with no community, and when they moved to somewhere with community they said that their English used to be better back there! I also saw how they became a burden on their children as they were not able to function properly in a country where they couldn't speak the language properly, closing off opportunity to themselves.
On the other side with Chinese students who come over to a western country, I found it frustrating that they were pretty shy & insular when speaking english while they were gregarious and cracking jokes in chinese amongst each other. I tried to connect, but I think they feel too embarrassed to try to speak english, so they don't practice enough outside the minimum and never reach competency and then stick with their chinese friendship group. And they don't seem that interested most of the time :(
You also see it happening with western expats in asian countries too.
So my suggestion to anyone moving to another country and planning to stay there for years, avoid the home group and force yourself to speak with mistakes. Otherwise you hurt and isolate yourself instead and write sour grapes essays like this one.