PBS’ Newshour marked the opening of The Hobbit by interviewing Jason Fisher, editor of Tolkien and the Study of his Sources: Critical Essays (a collection to which Diana contributed) —
How did language play into his creation of mythology?
Jason Fisher: Tolkien envisioned a whole series of different languages; languages spoken by elves, men and dwarves. Because he was actually trained in what would become historical linguistics, which was mainly called philology in his day, he was trained in how languages related to one another and changed over time. He attempted to mimic [that evolution] in his own creative world.
Tolkien’s interest in language led him to create languages, and he therefore wanted to create a mythology and a world in which those languages might have been spoken. For him it started with languages, with words, with names, and from those he created narratives and full stories.