I'm guessing just decide whether or not it's necessary in English. Here, "See you tomorrow, the girls," is definitely wrong. I'm not very far in French but I can tell you that in Spanish it is sometimes grammatically correct to include /or/ omit the article (translating from Spanish to English), in which case you would have to rely on context or just decide for yourself.
It could be that but it's more so a simple mistake because Duo listed "À" to mean "to" or "at"...so the sentence would be "At tomorrow girls" or "To tomorrow girls", which doesn't make sense in modern English.
So I said "See you tomorrow girls" since "À plus tard" means "See you later" but Duolingo still said I was wrong. They said the correct answer is "Til tomorrow girls" as if that is the literal translation. But in Google translate the "À" is omitted altogether, "les" is kept in, and it is literally translated as "Tomorrow the girls" which still doesn't make sense.
I just don't want to learn the wrong thing or be given a bad score for no reason lol, but thanks.