Newsweek this week began selling a Japanese-language edition of its U.S. magazine.

Introduction of the new magazine was postponed for nearly a year because of the problem of translating the English-language version into Japanese on a timely basis. The new magazine will have approximately 90 translators who will be able to produce the Japanese-language version two days after the English-language copies of the magazine go on sale here.

Financing for the new magazine venture is coming almost entirely from TBS-Britannica Co., a local company that has successfully marketed a Japanese-language edition of the Encyclopedia Britannica and is a subsidiary of Suntory Ltd., a major Japanese distiller and food processing company.

Although the magazine will seek a foreign image, it also is giving some ground to local conventions. Approximately 15 percent of its content will be written independently here. When possible, the magazine will emphasize Asian stories or others of special interest to Japan.

Newsweek, however, is not the first foreign publication to eye the Japanese magazine industry, which takes in almost $1 billion a year in advertising revenue. Cosmopolitan, Money, Playboy, Penthouse, Fortune and others are all here in some form.

Time, Newsweek's major rival, has studied creating a Japanese edition from time to time. "I admire the courage of Newsweek," said Hidehiko Kido, Asia managing director for Time Inc.'s Japan branch. "I wish I could say I expect them to succeed but, frankly speaking, the field is very difficult. That's why we decided not to do the same thing."

Newsweek is going Japanese just as the doyen of foreign magazines, the Readers Digest Japanese edition, is about to close shop. But Digest editor Ko Shioya said there is no lesson there that Newsweek will fail. The company was pulled down by unprofitable book and special-products lines, not by the magazine, which has a circulation of 460,000, he said.