In discussing the languages Tolkien invented, it is necessary to consider two aspects: their "primary world" history, namely their literal development by Tolkien as a linguist, and their "secondary world" history, namely their imagined historical development in the history of Middle-earth.
Tolkien was a professional linguist and a specialist in the Old English language. He was also interested in many languages outside his field and developed a particular love for the Finnish language (he described the finding of a Finnish grammar book as "entering a complete wine-cellar filled with bottles of an amazing wine of a kind and flavour never tasted before", The Letters of J.R.R. Tolkien, number 214).
Finnish morphology (particularly its rich system of inflection]) in part gave rise to Quenya. Another of Tolkien's favorites was Welsh