English is our first language most of the time. Some countries people learn english and speak it instead of their birth language for no other reason than reputation. They know their mother tongue, and they can speak it...so why do they speak english??? There are no english people walking around that they have to converse with....They do it for privilege.
There can be all kinds of reasons to speak a different language. In the Netherlands the posh people spoke French for a while. That is probably privilege. But, being able to think and converse in different languages can also help one think. There are people who use a different language than their mother tongue to process difficult decisions. Somehow it helps them to not get distracted in the intricacies of their own language, when making these decisions.
When it comes to subtle intimacies in language, nothing gets so close to me as the dialect I was raised in. Weird thing is, I do not even speak it well.