Joel Barlow, author of "Enticing Siam" explains: Thai for smile is yim, with variants (yim-yaam, yim-raraeng, rawy yim, yim-yoh!, prai-yim, yim-kram, yim-prai, yim-ka-ria-ka-raad, yim-jaeng, yim pen, yim-yong, yim lamai, yim haeng-haeng, yim huei huei, om-yim - my dictionary with the most had 13, but that's 15.). Lao and Pasa Neua (Kam Muang) have very similar words, I'm fairly sure. But another way to count would be to include Thailand's many languages of over 100,000 speakers: Yawi (the Malay word would also be similar, I'm similarly sure), Karen, Mon, Burmese, Khmer, Mandarin, Teh-chiu Cantonese, Lahu (Musur), Lisu, Akha, Hmong and Yao (Iu-Mien) - making 13 entirely different words! Lisu and Yao may have closer to 50,000 speakers each, but one can hardly count them oneself, and the government only reports how many are registered. There are also Moken Sea-gypsies, Suai, Lawa, Kamu, Htin and Mlabri Pi'i Tong Luang People of the Yellow Leaves, Farang and Japanese, but well, never-mind that.
Also: On the forum: Thai Language Urban Legend
Earlier: Urban legend? - June 4, 2007
This page: But did you also know that Thailand... has the following factoid: (Did you know Thailand) Has 13 different words for smile?
This sounds like an urban legend. Does anyone know?