The decline of the dollar and increased promotion overseas is proving profitable for the Florida citrus industry, which plans to spend $10 million this year advertising in Japan and Western Europe, industry experts say. After a record 1986-1987 season in which 22% of the state's fresh citrus was sold overseas for about $105 million, the industry is redoubling its advertising, said a Florida Citrus Packers Assn. spokesman. Analysts project a 4% increase in exports this season. Exports to nations around the Pacific rim are expected to jump from nine to 10 million cartons, while growth in European and Scandinavian sales is expected to rise by 1 million cartons, to 7.3 million.