I am speaking of course of the once-popular book GNOMES by Wil Huygen (text) and Rien Poortvliet (art), which had a vogue in the late '70s (and was much imitated) but is now I think pretty much forgotten.
I recently tracked down a copy and reread it for the first time in many years, as part of a larger discussion (still ongoing) I've been having with some friends about the origins of gnomes as a player-character race in D&D. It does not stand up well, but I was bemused to find that it does give a little 'Gnomish' in passing.
The first occasion is when we are told about mid-book (GNOMES having no pagination) that the Gnomes' word for 'goodnight' is slitzweitz.
The second occasion comes about a third of the way from the end, on a full page with the header 'Language':
Among themselves gnomes speak their own language.
But since we come in contact only with solitary gnomes,
we never hear it. (They can become very difficult
if asked about their language.) It is certain, however,
that animals understand it. "Goodnight" is slitzweitz,
and "thank you" is te diews. We did not progress much
beyond these few words mainly because the gnomes
master man's languages perfectly. And if they cannot
place a word, they immediately ask its meaning.
Their written language is the ancient runic script.
Beneath this is a picture of a gnome saying "Slitzweitz" = Goodbye
--a slightly different gloss from goodnight but no doubt close enough.
And that's it: I don't know if they made up more words in 'Gnomish' in the books that followed (only the first few of which I read, and that long ago -- definitely a case of diminished returns) but I thought it worth sharing that they at least made the effort. Though I suspect they were inspired more by Richard Adam's WATERSHIP DOWN than JRRT.
current reading: PRESIDENT FU MANCHU by Sax Rohmer (1936)**
*among Tolkien scholars, anyway.
**which I bought way back when working on the PULP CTHULHU project but have never read till now.