I will find this passage. I'm reading two other books right now, but I'll find it soon. I personally agree that it's interchangeable. But, like I said, I do remember something being mentioned in I think it was Unfinished Tales. I will find it.
Good luck! but the passages I recall (at the moment) that seem to imply a distinction are draft passages (at least one) for The Lord of the Rings and maybe some early 'Silmarillion' examples; and possibly an example in The Hobbit.
But in draft text Aragorn was once called Trotter; and in my opinion what matters is how Tolkien decided to ultimately explain the word 'goblin' and orc in published text (published by JRRT himself), reflecting his decision on the matter and arguably explaining all published instances.
Also, if I remember correctly, aside from Orcrist (which, like you said, orc translates to goblins), the word orc isn't used once in The Hobbit in reference to the "goblins." Definitely could be wrong, but I'm almost positive.
It occurs (in later editions) at least once outside of Orcrist, if I remember correctly.
Also isn't orc used far more in LOTR than goblins? I don't want to cause trouble with this, just a thought...
No trouble! I once counted the instances (that I knew of) of 'goblin' in The Lord of the Rings. I never counted the instances of orc, but I'm guessing there are plenty more! Yet according to Tolkien's published explanation, the amount doesn't really matter I think.
There has to be something out there that clarifies this.
In the 1960s Tolkien added a note to editions of The Hobbit -- thus it was added to editions published after The Lord of the Rings was on bookshelves, and thus after Appendix F 'On Translation' was in print.
Translation is the key! Appendix F had paved the way and all instances could be explained by using this key, or explained well enough anyway. In this note JRRT explained that orc is:
: not English
: a word used by Hobbits
: importantly, a word used at that time by Hobbits -- that is, used in Frodo's day
: usually translated 'goblin' in this book
You'd think they would be two different breeds of the same race, like dogs, but the fact it's used solely in The Hobbit to refer to what are obviously orcs throws off that logic by implying they are the same. I understand orc translates to goblin in elvish, but the story wasn't being told in elvish, it was being told by Bilbo, and even if it was that means he'd only be calling them orcs, so you'd think it'd be like LOTR where both were used. It just doesn't make sense!!! (just like that last sentence probably didn't to anyone else)
The story was being told in Westron as that was the language Bilbo, Frodo, and Sam spoke. The translator is Tolkien (so says the Elvish writing in the books), and it is only the modern translator who knows the modern word 'goblin', as no one spoke English in Frodo's day of course. Still, JRRT could have set up a scenario like he did with other words:
:kuduk translated 'Hobbit'
:kud-dukan translated (Old English) 'Holbytla, 'Hole- builder (or Hole-dweller)'
:banakil translated 'Halfling'
Even here note that a 'Hobbit' is not meant to be different from a kuduk. One word translates the other. Actually Sam (not his real name) would have no clue what a 'Hobbit' was, he would know the word kuduk rather. Anyway back to orc, in my opinion here's what we do not have:
:some unknown word -- translated 'goblin'
:a different unknown word -- translated 'orc'
Some seem to think this is the scenario, because they think 'goblin' translates a word used by Hobbits, and 'orc' translates another word used by Men. Not so, say I, the scenario Tolkien chose is simpler:
:orc translated 'goblin' ('hobgoblin' for larger kinds)
But both words are in the same story! It would be like using hund -- and sometimes 'dog' -- in the English translation of a German text! where one would always expect the word dog. With The Lord of the Rings we have an English translation of a largely Westron text, so we should really always, or at least mostly, expect 'goblin'. But JRRT decided otherwise after The Hobbit, although obviously he didn't go back and change all instances of goblin to orc in The Hobbit itself.
Tolkien even advises translators of The Lord of the Rings on the matter: although according to the system the word orc should be translated (like 'goblin' was usually employed in The Hobbit), since orc is fitter in Tolkien's opinion, he asks translators to simply leave it alone, despite the system -- in other words, he asks that translators do not translate orc with a word in the language of translation (Swedish or whatever).
So that part is unusual, I admit. Incidentally, Tolkien really didn't like 'Elves' as a translation because of its modern connotations, but he decided to stick with it.