A PJ inconsistency?

the inscriptions on the ring were tengwar adapted to the black speech. And true, the children recognized the character on the fireworks, but Frodo is much more educated than them. It is stated that he was the most learned of all the hobbits aside, perhaps from Bilbo. But none of that is even relevant to whether or not Frodo would know how to say "friend" in Elvish. He listened and understood the songs that the elves in Rivendell sang. He understood most of what was said in Lothlorien. He understood the balad of Beren and Luthien. Bottom line, Frodo knew elvish. And since the inscription above the door was in Elvish and read, "pedo mellon a mino", which translates, "Speak friend and enter", it doesn't make any sense that Frodo read it, and then didn't know what "friend" is. Merry wouldn't know the translation without being told, but Frodo himself translated as he re-read the inscription, commenting on it being a riddle. It wasn't written in the common tongue, so he translated from Elvish as he read, so he just finished reading "mellon" and saying "friend".