A PJ inconsistency?

yes, Gandalf did read the inscription, but when Frodo questions it, he also reads it. He is shown looking up at the door and reading, "speak friend, and enter." it is after he reads this that he looks at Gandalf and asks. Tolkien did not err in this, he had someone ask who did not know the language (Merry). Also, the two forms of Elvish (quenya and sindarin) evolved when some of the elves moved to middle earth, and some remained in valinor. those that moved slowly altered their speech, which then became sindarin. when the others followed hundreds of years later, the two had changed in time, to be nearly two separate languages, though both had the same root language. Also, in regards to iarwin's comment about frodo possibly not knowing the tenwar script, he tried to read it on the ring, remember? But because it was the Black speech, he didn't know it, but if he didn't know the script, he wouldn't have even tried to read it.