Royal Watch: Washington's most celebrated visitors will be wined and dined tonight on domestic champagne and imported caviar. Jeff Cohen, president of Sutton Place Gourmet Inc., said he sent eight pounds of Royal Chinese Sturgeon Caviar over to the White House at the request of Lady Marjory Wright and Nancy Reagan. Cohen said he was told that Charles and Diana will nibble on the $15-an-ounce delicacy as they watch the sun set over the Rose Garden.

Imported hors d'oeuvres may be okay, but the Reagans apparently don't want to be caught serving imported wines again. According to a spokeswoman from Schramsberg Champagne in California, the first couple ordered that all-American bubbly for the weekend. Hitler Diary Forger Exhibits His Work

Konrad Kujau, the self-confessed forger who produced the phony diaries of Adolf Hitler, is cashing in on his talent for imitation. A month-long exhibition of his work opened yesterday at Galerie Atlanta in Hamburg. "I need the money," Kujau told Reuter wire service.

Kujau and journalist Gerd Heidemann were found guilty in July of defrauding the West German magazine Stern of $3.6 million, the sum it paid for the fake Hitler diaries. Kujau admitted receiving $615,000 of the proceeds from the diaries hoax, but he and Heidemann say they do not know what happened to the rest of the money. The 47-year-old counterfeiter is free pending appeal of his 4 1/2-year jail sentence.

Kujau hopes to sell his copies of works by great artists at prices from $380 to $3,800. He also is offering a limited 100-copy edition of his illustrated "Life of Hitler," a musical record he has titled "I Was the Fu hrer's Forger," and as many fake signatures of the famous as his customers will buy.

His book "I Was Hitler," which tells how he forged the dictator's diaries, is due out by Christmas, and Kujau hopes to have his record released in English and Japanese. He plans to have the lyrics translated into Japanese and double-checked. "No one is going to pass a forgery off on me," he said. DA Rejects Reopening Monroe Case

Los Angeles County District Attorney Ira Reiner said yesterday that there isn't even a "bare suspicion" that actress Marilyn Monroe was murdered, and rejected a proposal for a new inquiry into her death. "As public prosecutors, we cannot support a grand jury investigation concerning matters of historical interest by artificially cloaking them in the guise of a criminal inquiry," Reiner said in a memo to the five county supervisors.

The actress was found dead in her Brentwood, Calif., home Aug. 4, 1962, of an overdose of barbiturates, according to an autopsy and a police investigation. Then-coroner Thomas Noguchi, who performed the Monroe autopsy, ruled her death a suicide, as did an investigation by the district attorney's office 20 years later in 1982.

Supervisor Mike Antonovich last month asked Reiner and the county grand jury to review a letter from Robert Slatzer, a consultant for the television news magazine "20/20," who claimed he was once married to Monroe and had new evidence about her death. Reiner said his office reviewed Slatzer's letter, but concluded it contained no new evidence that would support a third investigation. End Notes

Sen. Edward Zorinsky was released from George Washington University Medical Center yesterday after observation because of chest pains. A hospital spokesman said the senator's ailment was not determined, but tests showed he did not suffer a heart attack . . . Rep. Joseph P. Addabbo also was discharged from the hospital yesterday. Addabbo had been admitted to Walter Reed Army Medical Center Aug. 28 for an unspecified kidney ailment. . . .

Isaac Bashevis Singer, who was scheduled to read from his unpublished works tomorrow at Tifereth Israel Congregation, has had to postpone his appearance due to recent fainting spells. Singer, who won the Nobel Prize for literature in 1978, turned 81 last July . . .

Entertainer Sammy Davis Jr., 59, will undergo surgery Monday to reconstruct a hip, a spokeswoman announced Thursday. Davis, who was injured while filming "Alice in Wonderland" for television, is reported to be in excellent condition . . .

Candice Bergen, who has worked as a model, photographer, political activist, magazine writer and movie actress, yesterday added motherhood to her list of accomplishments, giving birth in New York City to a 9-pound, 2-ounce girl. The daughter, named Chloe, is Bergen's first child; her husband, film director Louis Malle, has a son and daughter by a previous marriage . . .