(1)Fixing a car is a challenge.
(2)Fixing cars is a challenge.
(3)Learning a language is hard.
(4)Learning languages is hard.
I am not an English native speaker, but I feel like, (1) and (2), both of them mean fixing a car is a challenge even though "cars" is plural in (2). I don't think (2) is saying that "fixing two or three cars" is hard, or at least (2) is not emphasizing the case of fixing many cars.
And I think (3) and (4) also mean the same thing: "learning a language" is hard.
I think, both of the plural and singular forms are used as some sort of general speaking and can be translated to "learning languages is hard no matter how many languages you learn" and "fixing a car is a challenge no matter how many cars you fix." Am i right?? Please tell me what you think about it. Thank you.