Welsh most strongly influenced the sound of Tolkien’s invented language Sindarin. Tolkien admired Welsh phonology and used it as a major model for the language of many Elves in Middle-earth.
Sindarin developed from Tolkien’s earlier ideas about an Elvish language called Gnomish. Within the fictional history, it is the everyday Elvish language of many Sindar and becomes widely used in Middle-earth. Its words and grammar were constructed rather than copied directly from Welsh.
Finnish was another major influence on Tolkien’s linguistic work, especially on Quenya. This distinction causes a frequent mix-up: Quenya is commonly associated with Finnish, while Sindarin is commonly associated with Welsh. Tolkien’s language-building drew on several real languages without making any of them a direct translation key.