After a campaign of sabotage and civil disorder two years ago that included a threatened fast until death by a leading politician, Wales this week got the dream of local nationalists--a television station all its own, called Sianel Pedwar Cymru or Channel 4 Wales, also known as S4C.

In the razzle-dazzle big-money game of international broadcasting, it is a fair bet that the appearance of S4C in this picturesque corner of the globe did not cause much of a stir. Yet the station does represent an important video venture for Britain: an effort to sustain an ancient, and now endangered, language by giving it the powerful boost of a prime time television outlet.

As its managers see it, S4C is a major network in miniature. For an average of 22 hours per week, it will show specially produced Welsh-language programming that will be original material commissioned and produced locally and starring Welsh performers. The rest of its programs will be in English, taken from Channel 4, a new commercial service that also premiered throughout the country this week.

The Welsh programs are broadcast from about 6:30 to 9:30 p.m. and are designed to be popular with viewers and attractive to sponsors. There will be soap operas, adventure dramas, situation comedies, cartoons, sports, music, public affairs and a half-hour of news each night. Last week, a Welsh-speaking correspondent was dispatched to Washington to interpret the U.S. midterm elections.

A start-up budget of $34 million has been plowed into spiffy offices in downtown Cardiff, state of the art technology, lavish promotion and ambitious programs. The station mascot, a cartoon character named Superted (a courageous teddy bear) already is being marketed in dolls, T-shirts and other paraphernalia, with foreign-language rights sold in 30 countries based on previews at television festivals, according to S4C's director, Owen Edwards.

THERE ARE EVEN such big-time problems as hefty cost overruns. The producer of a planned 90-minute saga about an aristocratic woman of some period in the past who doubles as leader of a band of highwaymen was fired when the project almost doubled in cost and length. It was salvaged by its author, Dafydd Huw Williams, who now sees it as a two-part series.

What makes all this so unusual is that the potential audience for S4C is tiny by today's standards. A hundred years or so ago, 90 percent of the people in Wales spoke Welsh, a Celtic tongue (or tongue-twister for any outsider who tries to speak it) that already was well-established when English interlopers arrived in the 6th century.

Today, Welsh-speakers account for only about 20 percent of the population.

Moreover, Wales already is served by the two national British Broadcasting Corp. channels and one commercial channel that are bound to attract the bulk of available audiences with what a Welsh nationalist derisively called their "Anglo-American" programming. At any one time, therefore, there may be no more than from 50,000 to 75,000 viewers for S4C's Welsh output.

IN SOME AREAS, there is resentment over the broadcast of Welsh programs instead of more English ones. A recent survey of subscribers to Rediffusion, a cable company operating in the "Valleys" region of Wales, showed that nearly 90 percent of those who responded favored getting the all-English Channel 4 over S4C. The government ordered Rediffusion to carry S4C anyway.

But to the staff at S4C, the available audience, small as it may be, is an essential base for preserving--and perhaps invigorating--the Welsh language and culture. Public relations officer Ann Beynon, for instance, said that Welsh was her first language at home and as a child she learned English by watching television.

"Why shouldn't the process work the other way around?" she asked.

IN ANY CASE, the impact of the station on Wales will be measured by the government in three years, which poses a formidable challenge to S4C, as Edwards acknowledges.

"Unless our service is seen as relevant and attractive to Welsh speakers, they won't watch," he said. "They certainly won't look at programs just because they are in Welsh, nor should we expect them to."

S4C is one of the world's few television stations that has its roots in organized civil disobedience. After the Conservative government of Prime Minister Margaret Thatcher took office in 1979, it reversed an election pledge to give Wales its own channel.

The result was a campaign of sabotage against television transmitters by nationalist groups. More than a dozen people, including some senior academics, were jailed. Two thousand persons refused to pay their annual television license. Then, Gwynfor Evans, president of the Plaid Cymru, the Welsh nationalist party, announced that he would fast until death over the issue.

The government, believing that violence was inevitable if Evans went ahead with his threat, reversed itself and promised that the station could go ahead. "This is the biggest victory we have ever won for the Welsh language," Evans exulted. Now with S4C finally on the air, that declaration is being put to the test.