But that's a really bad translation, out of context! "A century" is a hundred [thing, depends on context], and by itself is fairly meaningless.
Duolingo often has the problem with trying to translate very short passages into English, which is a very context-sensitive language. What it should probably be doing is translating longer sections....
I wonder if there is a benefit of some sort having multiple definite forms in a language. I never missed having another one while learning english as a german native. Well, in germany the third one "die" is used for plurals, however, some words use "die" even if it is a singular, those words are marked as feminin - which is growing to be kind of a problem of its own now with the ongoing alternation of languages in regards of equality.
Disambiguation, mostly. Take a sentence like "I went to the store and bought a box when I saw how big it was". Native speakers might assume "it" refers to the box, since it's later in the sentence, but it's a bit ambiguous.
But, if store and box belong to two different definite forms, then there's no ambiguity. We still have this in English when dealing with human pronouns; a written (or otherwise conveyed, say relayed verbally) conversation is a bit easier to follow when it's between two people of different genders.