Which Tolkien language was chiefly inspired by Welsh?

Sindarin is the Tolkien language chiefly inspired by Welsh.

Tolkien developed Sindarin as one of the principal languages of the Elves in Middle-earth. Its sound patterns and some of its linguistic character were strongly influenced by Welsh, a language Tolkien admired. Sindarin is associated especially with the Grey-elves, or Sindar, and became a common spoken Elvish language in much of the Third Age.

Quenya, another major Elvish language, was instead influenced largely by Finnish, Latin, and Greek. Tolkien did not merely create isolated words: he developed historical relationships, sound changes, grammar, and writing systems for his languages, giving them the impression of long linguistic histories.

Sindarin names include Lúthien, Galadriel, and Minas Tirith. Tolkien’s linguistic work began long before his novels were published, and the languages often helped generate the peoples, histories, and names that make Middle-earth feel internally coherent.