Amnesty said Mr. Tashi was a “prisoner of conscience, detained solely for exercising his right to freedom of expression.”

Human Rights Watch released a detailed chronology of the case and called it “politically motivated.”

“Tashi Wangchuk has joined the ranks of those prosecuted in China by simply calling for rights to be respected and the law to be upheld,” said Sophie Richardson, the group’s China director. “Cultural rights, which include the right to use one’s own language, are protected under both the Chinese Constitution and international human rights law.”

Mr. Tashi’s case has drawn intense international attention. In December, Max Baucus, the United States ambassador to China, mentioned Mr. Tashi and several other political prisoners in China in a long statement emphasizing the importance of human rights. PEN America, a group that promotes freedom of expression, noted Mr. Tashi’s detention in a 76-page report in September that criticized China’s attempts to censor foreign reporting.

A campaigns director with Students for a Free Tibet, based in New York, said Wednesday that the group planned to ask its members to publicly advocate for Mr. Tashi.

Lawyers for Mr. Tashi were allowed to visit him briefly last year in a Qinghai detention center. Mr. Lin said Mr. Tashi was in good spirits, given the circumstances, and was writing up points to make in his defense during a trial. When interviewed by The Times in 2015, Mr. Tashi insisted repeatedly that his remarks be on the record and said he knew he could be imprisoned after publication of the video and articles, given the political sensibilities in China over Tibet.

Mr. Tashi traveled to Beijing in 2015 to look for lawyers to help him file his lawsuit and to try to get China Central Television, the state network, and foreign journalists to document his efforts and the lack of robust Tibetan-language education in public schools in Tibetan regions.

Mr. Tashi said he wanted to use Chinese laws to expand language education, and he praised President Xi Jinping for having “promoted a democratic and law-abiding country these last few years.”