Most people who learn a second language to a reasonable level learn about register. Actually, almost everyone who learns to speak learns about register, in the sense that they learn to be implicitly aware of it. But people who learn to speak another language to a high enough level usually learn about it explicitly.
For those who don’t know about it explicitly, register is a very simple concept: it means that you don’t use language in the same way in every context. You don’t, for example, talk the same way to your mates down the pub as you do to potential employers at interviews. The concept is simple, but mastering the skill is actually quite complex. Just as you need to keep on learning new words throughout your life, you need to keep on mastering the details of register; and getting the details right is tricky. First-year undergraduates tend to get the details wrong a lot when they write essays; eventually some of them learn to get it right. Similarly, people who haven’t spent enough time socialising to get used to the conventions often get them wrong when they do socialise, and they sound very formal to most people.
It’s worth saying a brief word about what it means to be “wrong”. In this post (and here and here) I compare language to clothes: you can be wrong in your use of language, but only in the sense that you can be wrong in your use of clothes. Different social contexts have different dress codes, some of them very explicit, and some of them more implicit (though this doesn’t mean that they’re not very strong); contexts with no dress code at all are rare, although contexts vary considerably in how much they constrain how you can dress. It’s much the same for language.
There’s probably not much in the above that will surprise readers, or be very new to them. However, I’ve come across more than one person recently saying things like:
The difference between these two ways of speaking isn’t very big; it’s just a matter of register!
Just a matter of register? Just? This implies a deep misunderstanding, but one that I suspect is very common. To put it in perspective, consider the following: in some places, some contexts dictate the use of one language, while other contexts dictate the use of an entirely different and unrelated language. This is a matter of register. If you use Swahili in some contexts and French in others, then this is a matter of register. It is not, incidentally, all that uncommon for such situations to arise. Consider the complicated relationships in India between the indigenous languages and English (it’s not even the case, as it happens, that all the indigenous languages are related to each other).
There is, of course, a related phenomenon of which we’re all aware, but which tends not to be called register, mainly because it’s geographically, rather than socially, conditioned, and because no one can be expected to master more than a tiny fraction of its details. A simple case of it is that you’re generally better off speaking French in France and German in Austria than you are speaking Welsh in either place. As in cases of what we normally call register, of course, contexts can conflict; imagine if you were invited to a party in France hosted by the Welsh-language society of Austria. Moreover, English is becoming more acceptable in more and more contexts internationally (and, as a result, less and less acceptable in some). In this, it behaves rather like a standardised variety in a particular country. It’s probably the case that you’ll do less badly on the whole sounding too formal all the time than you will sounding too casual. Experience has suggested to me that the same goes for clothes.