I think the answer is that English is unusual in how often we use nouns without articles. Most other languages I've looked at (French, German, Portuguese, Latin) tend to use them more, even in situations when English doesn't. So, there will be lots of translations from other languages to English when you don't really need to translate the article, since English wouldn't use it in that situation. This is a perfect example of that - Italian requires you to use "il sale" but when it comes out of the English translator, the article disappears and you just have "salt." I think what is more bizarre than these little differences is that we can translate from one language to another at all! Languages are so different that it sometimes feels like translating is like to trying to explain architecture to someone through modern dance. What dance move translates to "Art Deco"?
Whether or not we use the definite article for salt, it's odd to say "The ingredient is (the) salt" because we assume that there are multiple ingredients because you can't make anything edible with just salt. This sentence would probably be better translated as "The secret/special ingredient is (the) salt." If we say "the salt", we imply that it may be a special kind of salt, like seasoned salt or Jane's Crazy Mixed-up Salt.
It can also mean that.
Just as English "like" can mean either "have good feelings towards someone or something" ("I like beer") or "similar to" ("it looks like water") - two completely different meanings that just happen to be spelled the same.
You can tell them apart because one's a verb and the other's a preposition, so they wouldn't occur in the same place in a sentence.
Similarly, "sale" can be either a verb ("it goes up etc.") or a noun ("salt"). Depending on how it's used in the sentence, you can tell which is meant.
La is feminine used with feminine words, such as la zuppa; the soup. il is masculine used with masculine words such as il gatto; the cat. Un is also masculine used with masculine words such as un cane; a dog. Una is feminine used with feminine words such as una bicicletta; a bike.