In the Harptos Calender, every month has a common name. But Hammer and Flamerule already seem to be English. Would you translate them into another language (Italien, German, etc.) or keep the official names like that? In 1e/2e D&D, the Common Tongue in the game was design-considered to be English. Hammer and Flamerule were the Common names for "Hamaer" and "Flaenrûl," respectively, and had 'taken over' in Faerûnian daily usage.#Realmslore

— Ed Greenwood (@TheEdVerse) October 17, 2019

So I'd either leave all the names as is, or translate if it works better for your purposes. Clarity and a medieval/Renaissance fantasy setting 'feel' are the goals; for your gamers, what works best has to be your call. :}

— Ed Greenwood (@TheEdVerse) October 17, 2019