In the chapter A Journey in the Dark in Lord of the Rings: Fellowship of the Ring, the Company comes to the western gate of Moria, the Doors of Durin, which is sealed and requires a password to be opened. The door carries an inscription that in the book is depicted as follows:
As the picture says, the text inscribed is:
Ennyn Durin, Aran Moria: pedo mellon a minno
Im, Narvi, hain echant; Celebrimbor o Eregion teithant i thiw hin
The Doors of Durin, Lord of Moria. Speak (= say) friend and enter
I, Narvi, made them. Celebrimbor of Hollin drew these signs.
Now that is clearly Sindarin. In fact, it seems to be very bog-standard Sindarin, with no unexpected features. Yet Frodo cannot read it.
That is perhaps not entirely unexpected, since it does clearly say in the caption to the illustration that the inscription is written “in the Feänorian characters according to the mode of Beleriand”, so it could well be like seeing English written using the Russian alphabet to Frodo. (There are indeed clear differences between how this inscription uses the Tengwar and how they are more commonly used in the Third Age.)
But: when Frodo says he cannot read the letters, Gandalf replies:
The words are in the elven-tongue of the West of Middle-earth in the Elder Days. [my emphasis]
In other words, the words aren’t just written in a different alphabet, they’re in a different language. But what language?
As far as I know (and I may be wrong here), all the Ñoldor had adopted (dialects of) Sindarin long before the Second Age when this inscription was made, and the illustration (supposedly taken straight from the Red Book) is in Sindarin also. Frodo knew a fair bit of Sindarin, and considering the entire Company had just come from Rivendell where it is the native language (more so than Westron), it would be very odd if Gandalf referred to Sindarin in such an odd, roundabout way, rather than just calling it Sindarin.
The accepted answer to this related question suggests that the anachronistic use of the name Moria in this inscription may be due to the scribe/illustrator of the Red Book having substituted the older Elvish name Hadhodrond for the more commonly used and widely known Moria; perhaps, then, the entire inscription has been translated into Standard Sindarin from whatever language (presumably closely related to Standard Sindarin, at the very least a common descendant from Old Sindarin) it was originally written in.
Has canon ever addressed this? Did Celebrimbor not speak Sindarin natively?1 What is this “elven-tongue of the West of Middle-earth in the Elder Days”, then, if not Sindarin? Alternatively, was Gandalf just being inaccurate when he used the term “elven-tongue”? Should he have said “written in the script/characters/style of Beleriand in the Elder Days” or something like that instead?
1 I’m aware we don’t know much about his early years, nor even if he was born in Valinor or Middle-earth. The name Celebrimbor is Sindarin, but may not have been the form he actually used himself, similar to how Virgil did not call himself Virgil but Vergilius.