Why is "A disznók" translated into English as just "pigs" without a definite article? Aren"t articles used equally in English and Hungarian?
No, articles are not used identically in English and Hungarian. The definite article is used far more frequently in Hungarian for general statements like this.
To be honest, though, we do the same thing in English... sometimes. If someone says, "The pig is a lazy animal" (or "The cow is a useful animal"), it's probably a statement about pigs (or cows) generally, rather than a specific pig. In English I think we tend to either use the definite article with a singular noun ("The pig") or a plural noun with no article ("pigs") to make the general statement.
Personally, I would have expected the singular (A disznó) in the Hungarian here, too.
Practically speaking: If you're translating the general English sentence "Pigs are lazy animals / The pig is a lazy animal" into Hungarian, then use the definite article.
If you're translating the Hungarian sentence A disznók lusta állatok into English, it might be translated as general "Pigs are lazy animals" or it might possibly be specific "The pigs are lazy animals." You'd need some context to tell the difference.