Welsh was a language spoken in Wales.
In Cardiff on 24 December 1869, after buying a newspaper, the seller wished the Ninth Doctor a "Nadolig Llawen", a traditional Welsh Christmas greeting. (TV: The Unquiet Dead)
In 2006, Blon Fel-Fotch Passameer-Day Slitheen, posing as the Lord Mayor of Cardiff, Margaret Blaine, chose the name Blaidd Drwg for a nuclear power plant planned for construction in central Cardiff. According to Blon, she chose the name at random, thinking it sounded good. The Ninth Doctor noticed that Blaidd Drwg was Welsh for Bad Wolf, another occurrence of the Bad Wolf meme. (TV: Boom Town) When Donna Noble asked the Tenth Doctor what "Celtic" meant on 23 August 79, he explained that it meant "Welsh". (TV: The Fires of Pompeii)
"Caerdroia" was the Welsh phrase for "the fortress of many turnings." (AUDIO: Caerdroia)
Tegan Jovanka believed that her first name was Welsh in origin. (PROSE: The Immortals)
The Tenth Doctor claimed that he had never learned to speak Welsh. He had a pocket Gallifreyan-Cymraeg phrasebook with him while visiting Arcopolis. (PROSE: The Eyeless)
Both Gwen Cooper and Rhys Williams had poor knowledge of Welsh. (AUDIO: Forgotten Lives)