Limbile vechi sunt bune (:
Do you really need the article here? I feel the Romanian sentence could translate as Old languages are hard, even though in Romanian you express generality with the definite form...
You definitely don't need the English definite article here. If you're expressing a generality, it's always omitted.
I join in this question, but also wonder why "grele" sounds like it is pronounced with three syllables. Oh, perhaps it is because the "r" is rolled!
It's only two syllables. The rolled r sound is pretty difficult for English speakers, but once you get the hang of it, it shouldn't add any extra syllables
I know this is the unit about articles but Duolingo should let you omit the article in English, to stay true to the language
Does this sentence mean that old languages are hard to learn?
Yes, it does. Since "grea/grele" can also mean "heavy," you could also use "sunt dificile" ("are difficult") or "dificile de învățat" ("difficult to learn"), which can remove some of the ambiguity.