A spokeswoman for the State Department, Heather Nauert, said: “We are deeply disappointed that the Chinese government convicted Tashi Wangchuk.”

The Chinese Communist Party for decades maintained policies intended to keep ethnic minorities, especially Tibetans and Uighurs, under political control while giving them some space to preserve their own languages and cultures. But under Xi Jinping, the staunch Communist Party leader who came to power in 2012, China has adopted more assimilationist policies, designed to absorb these minorities into the fold of one Chinese nation.

At his trial in January, Mr. Tashi, speaking in Chinese, rejected the idea that his efforts to rejuvenate the Tibetan language were a crime. He has said that he does not advocate independence for Tibet, but wants the rights for ethnic minorities that are promised by Chinese law, including the right to use their own language.

After Mr. Tashi’s trial, six experts advising the United Nations on rights said, “We condemn the continued detention of Mr. Wangchuk and the criminalization of his freedom of expression.”

They added: “Free exchange of views about state policies, including criticism against policies and actions that appear to have a negative impact on the lives of people, need to be protected.”

Mr. Tashi studied for three years in a Buddhist monastery, and taught himself to write Tibetan with the help of a brother. He ran a shop in Yushu that sold local goods in person and online, and was shown in a promotional video in 2014 for Alibaba, the giant Chinese internet trader, as an example of entrepreneurial success.

Yushu, called Gyegu in Tibetan, is one of the predominantly Tibetan areas of China that lie outside the official Tibetan Autonomous Region, and the government has encouraged increasing use of Chinese language in schools and official settings across these areas.