Since English doesn't have a distinction between you singular and plural anymore, we sometimes add extra words to convey the plural. The all is optional in the English translation.
Just pointing out that in previous sentences, in the Latin version, the adjective Latin(-a, -as, etc.) has been capitalized, but here it isn't. ""Litteras latinas scribitis." What is the rule?
"Littera" can also mean a letter. Thus "You write Latin letters" should be accepted, too.
Can 2nd person plural "you" also be a polite formal singular "you" as in French & German?
No, such things developed during the Medieval period and I don't think it ever applied to Latin.
Is "You all write Latin literature" grammatically correct vs "All of you write Latin literature"? Just wondering...
Both are grammatically correct in English, but "you all" (or "y'all") is the more typical second person plural. If you say "all of you" you emphasize the idea that what you are saying encompasses and applies to every single person you are speaking to.
I think You write latin letters and You write letters in Latin should be accepted.