It depends on tne context. If you were talking about various nationalities you like or dislike then no, the 'the' is unnecessary. However if you're in a group that consists of various nationalities then you would use 'the' to distinguish them from the other people in the group.
It would be normal to say "I like Americans" or "I like Australians" (meaning all of them) . Saying "I like THE Americans" is perhaps more likely to be used in a particular context to refer to a specific group of Americans but can also refer to all Americans. So on that basis I think "I like Swedes" is fine. However saying "I like French" doesn't really work, partly because it lacks an "s" at the end to indicate that you are talking about multiple people and because it can refer to something else entirely. In this case you need the "the" or to say "I like French people". [Australian NES]