Comment by KineticLensman

> Tolkien was a seasoned linguist and he worked on LoTR for about a decade

This actually understates the effort Tolkien put in. He'd started the world-building that led to LoTR approx 35 years before the publication of the first volume (in 1954), specifically by writing the first tales in the Legendarium we now recognise as The Silmarillion. And he never actually completed the latter even having spent almost 60 years working on it.