J.R.R. Tolkien based the sound and structure of Sindarin mainly on Welsh when creating it for Middle-earth.
Sindarin is the language of the Sindar, or Grey Elves, and became the everyday Elvish language of many Elves in Tolkien’s later legendarium. It is one of Tolkien’s most developed invented languages, with vocabulary, grammar, sound changes, and historical relationships to other Elvish tongues.
Tolkien’s linguistic inspirations were not simple one-to-one copies. Welsh influenced Sindarin’s phonology and overall character, while Quenya is famously associated with Finnish and Latin influences. Sindarin and Quenya belong to the fictional Elvish language family Tolkien called Elvish or Eldarin languages.
Readers sometimes use “Elvish” as though it named one language, but Tolkien created several. Sindarin is also distinct from the languages of Dwarves, Men, and Orcs, each of which has its own place in Middle-earth’s linguistic history.