Many of students need to learn a new language to impove their job prospects but some of them are learning a new language purely for pleasure.
Usually, to indicate that it is an event happening at the moment of speaking. If you said "some (of them) learn..." it would not necessarily indicate this. This would usually mean a habitual action, just like the first use of the simple present (need to learn).
But without more context, or knowledge of under what circumstances or for what reason this statement is being made, all we can give you are generalities (it probably or usually means this).
The meaning would be different. "Some of them learned a new language" means that they did it in the past, and the process of learning the new language is now complete. "Some of them are learning a new language" means that they are learning it now. They don't know the new language yet: they are still working on it.
I suppose you could say that in practice, learning a new language is an on-going process, it would be hard to say when you were truly finished. The distinction would be more meaningful if you were talking about a discrete event, like "some of them took Ruritanian 101 last semester" versus "some of them are taking Ruritanian 101 this semester". But still, if someone has taken just one or two classes in a foreign language or has just read the first chapter of a book, he presumably only knows a few introductory words and phrases, you might well say he "is learning" the language. Whereas someone who has studied it for several years and can talk comfortably with native speakers "learned it" at some time in the past.