Des sacs is some bags. It is not bags in general. It is not all bags but some bags. More than one but not all.
les sacs = those bags right there
des sacs = some but not all bags.
les sacs = all bags, all examples of bags, the idea of bags, all the bags I've ever seen or heard of, all members of a collection of bags.
In English we say I like bags to indicate we are talking about all bags. We just drop the article. But in French, you can't just drop the article whenever you feel like it. There has to be a modifier for most nouns. So, the French gave le/ la/ les the dual role of referring to all examples of something.
Hence English Bags becomes les sacs because we have to have an article to show that we mean all bags. Les sacs becomes bags because in English we just drop the article and leave it to the listener to figure out whether we are talking about particular bags, some bags or all bags.
How do you know which meaning of les is intended here? You don't without context which is not present. Which is why Duo accepts either bags or the bags as a translation of les sacs. It could be either. But it definitely isn't some bags because the it isn't des sacs. Conversely it isn't des sacs because it isn't some bags.
Les sacs is either those bags right there; or it's all bags,
Duo lingo needs some work - the phrase "les chaussures" was given. I was to provide the English translation. I typed "Shoes". I received a "wrong answer". However the next exercise was the one where a phrase is given and you select the correct words from the choices given. The phrase was Les sacs. Only "Bags" was a choice, not "The bags". So to avoid confusion, Duo lingo needs to be consistent.
Duolingo cannot show you only one article for one grammatical case because it would exclude all others.
- the bags = les sacs
- bags = des sacs
- some bags = des sacs / certains sacs
These changes depend on context. But in any case, if you are given "the something", the French will have definite article le, la l' or les.
You can drop the some/des when translating French to English as long as you understand that it is there. But when translating English to French you have to translate the absent some into des.
If you routinely drop the translation going in one direction it seems to me that will encourage you to forget to include it going the other direction.
In English, we leave it up to the reader/listener to figure out from context which article applies if needed. But in French, you have to have a modifier of some kind for most nouns. The obligation is on the writer/speaker to clarify the context not the reader/listener.
English speakers don't care because most times it is not important and if it is, the listener can try and figure it out or just ask the speaker.
If it is "les xxx" (without anything else), you could say "the xxx" or "xxx". If it is "des xxx", you could say "some xxx" or just "xxx". All the nouns will be plural. The point is that, depending on what you put in, DL may show "another correct translation". That means that your answer is okay and here's another correct answer. Just because it shows a different answer doesn't mean that your answer is wrong (unless it is actually scored as wrong).
Without context les sacs/the bags could be taken to mean all bags, all examples of bags, the idea of bags. To express that same notion in English, you drop the article.
I think bags are good could be taken to mean that you think all bags are good.
Without context les sacs/the bags could be taken to mean those bags right there, the ones that we know about, the ones that we were talking about. To express that notion in English you include the article.
I think the bags are good would be taken to mean you think those bags are good.
Duo didn't give you any context in their example so you would be correct to include the article or drop it when translating to English. Unfortunately, a casual disregard for the article doesn't work when translating from English to French.