Sindarin was the Tolkien language primarily inspired by the sound of Welsh.
Tolkien developed Sindarin as the language of many Elves of Middle-earth, especially the Sindar. Its phonology and appearance draw strongly on Welsh, one of the real-world languages Tolkien admired. Sindarin appears in names, poems, inscriptions, and dialogue throughout The Lord of the Rings and related writings.
Sindarin is often confused with Quenya, another major Elvish language. Tolkien associated Quenya more closely with Finnish in its sound and structure, while Sindarin served as the commonly spoken Elvish language in much of Third Age Middle-earth. Neither language is merely a vocabulary substitution: Tolkien created historical relationships, sound changes, and grammatical development to make the languages feel like parts of a real language family.