"I just would not move to another country and not learn their language. The idea would be to make my life easier, not go out of my way to make the country I am living in change to suit me.
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Sure. I won't even VISIT another country without learning enough of the language there to be polite, ask common questions, count, and order a meal. It doesn't take that long, but then, I seem to have a knack for languages.
That's not always the case with recent immigrants. English is tough to learn. I'll bet these workers can speak it well enough to handle their duties at the cash register, etc. But, when it comes time to discuss the coming marriage of their daughter, or their last trip to the doctor, it's often easier to speak in their native language to others who also speak that language.
This is not about interaction with customers. I'm sure the owner of the store is happy to have his clerks able to speak to his customers. This is about interactions between employees, if it's like most of these cases.
It's a lot different to tell a customer how much their purchase comes to and to wish them a nice day than it is to have a full conversation with a co-worker who happens to share a native language with you. It's only natural to carry out those conversations in the native language.
Yes, immigrants should learn English, and almost all do, at least well enough to get along. But, first-generation immigrants will continue to address people who speak their native language in that language. It's a lot easier and more information gets exchanged.
I'm just not an advocate of insisting that every immigrant be fluent in English the minute they step into the USA. That's impractical, and has never been the rule here.