As I understand it, the first & the second sentences are correct and common. What about the third and the fourth sentences, especially the third one? Is it good to use "it's" if after it are several subjects? What is the alternative if the third sentence is wrong?
It’s easy to learn Spanish.
Spanish is easy to learn.
It’s easy to learn Spanish and Italian, especially one after another.
Spanish and Italian are easy to learn , especially one after another.