Re: Gandalf and Glamdring
An internal answer could be that the Silmarillion is a history recorded and compiled primarily by Elves who had long taken Sindarin as their primary language. (And perhaps tranlated into Westron as a document, Translations from the Elvish, written by Bilbo, who would probably be more familiar with Sindarin than Quenya.) A similar argument could perhaps be made for Bilbo's diary (i.e. The Hobbit) itself. While not written in Sindarin, it's reasonable that Bilbo would have used the proper names most familiar and more closely related to Westron.
Originally Posted by Morthaur
An external answer is that for a long time, the prototype of the language we know as "Sindarin" was formulated by Tolkien to be the birth language of the Noldor in Valinor. Only late in the development of the mythology did Tolkien decide / realize that it was instead the native language of the Sindar, and only the adopted language of the Noldor in Middle-earth. So even if Tolkien had intended to give us the native names of the Noldorin princes, it's possible this was never thoroughly integrated into texts from which his son had to compile The (real world) Silmarillion
Interestingly, since this language shift was long after the writing of The Hobbit, it also implies that the names inscribed on to Glamdring and Orcrist were intended to be some sort of Noldorin even though they're given to us in Sindarin.
Logical, maybe, but there's no hint of this in the stories. Except in a very tenuous way, when toward the end of his life Tolkien theorized that Galadriel tried to make her way to Middle-earth before the darkening of Valinor.
Presumably the Sindar from Aqualonde were able to sail back to middle earth prior to the years of the sun and the moon as they seem to have done when the visited Numenor in the 2nd age, thus Daerons' runes may have arrived in the undying lands prior to the departure of the Noldor, although sea voyages for any elves from middle earth in the first age seemed to be problematic, but if this was the case then why did the Sindar at Aqualonde build a large ocean going fleet?.
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