I read the glossaries of the Lord of the Rings books and it said that the language spoken by the Dwarves was not spoken to others except for a few words. That got me wondering if Tolkien had ever even invented that language as a whole. Did he invent Dwarvish far enough to carry a conversation or not?
There simply aren't enough words in Tolkien's Khuzdûl to carry on a conversation. This is noted by Christopher Tolkien in History of Middle-earth, when remarking on the use of the language in the inscription on Balin's tomb:
The use of the Dwarf-tongue (Khuzdul) is possible in so short an inscription, since this tongue has been sketched in some detail of structure, if with a very small vocabulary.
History of Middle-earth XII The Peoples of Middle-earth Chapter X: "Of Dwarves and Men"
It's worth noting that the Khuzdûl we hear in The Hobbit film trilogy was constructed by David Salo, a professional linguist with a passionate interest in Tolkien's languages. He discusses some of the challenges of this task on his blog, which is worth a read if you're interested in this sort of thing:
When I was asked to come up with some Dwarvish-language lines and lyrics for The Lord of the Rings, I initially balked. It wasn't my first experience with constructing Khuzdul — I had invented some names for the Middle-earth Role Playing Game several years earlier — but that had been with the understanding that I was, in a sense, contributing to a new world, related to Tolkien's but not quite the same. This felt a bit different. I pointed out that the amount of written Khuzdul could fit on a couple of pages (this is still basically true) and that almost nothing was known about its structure. I said that whatever I wrote in it would be largely a new invention, and that I wasn't going to pass it off as Tolkien's own work. I got the go-ahead anyway, and plunged in.