I'm a great fan of Tolkien's books, especially The Lord of the Rings and The Hobbit. One thing I have always wondered about in his books though, is whether the Elvish tongues were actually fully new languages. Were they totally constructed as it were from scratch, or was Tolkien patterning them after some European / other languages that were actually in use (or ancient languages out of use)?
Tolkien started really early in his life to develop languages. Started at 13 and developed languages till his death.
Tolkien was a professional philologist 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".
Finnish morphology (particularly its rich system of inflection) in part gave rise to Quenya. Another of Tolkien's favorites was Welsh — and features of Welsh phonology found their way to Sindarin. Numerous words were borrowed from existing languages, but less and less obviously as Tolkien progressed. Words that are an exact match with existing Welsh words can be found in the early drafts of Tolkien’s manuscripts published as The History of Middle-earth, but attempts to match a source to a particular Elvish word or name in works published during his lifetime are often very dubious.
Source: Languages (Tolkien Gateway)
As mentioned on Wikipedia, it was modelled on Welsh and some other Norse languages:
Sindarin was designed with a Welsh-like phonology. It has most of the same sounds and a similar sound structure, or phonotactics. The phonologies of Old English, Old Norse and Icelandic are also fairly close to Sindarin and, along with Welsh, certainly did have an influence on some of the language's grammatical features, especially the plurals (see below).