FIFTY people, including 15 foreign prostitutes, were arrested yesterday during raids on illegal massage parlours operating as brothels. About 280 policemen raided villas and parlours in Shamshuipo, Mongkok, Kowloon City and Yau Tsim. One of the prostitutes was from France and others from Thailand, Malaysia, the Philippines and China. A police source said it was unusual to arrest a French woman during a vice raid and they were investigating whether a prostitution racket had arranged for her to work in Hong Kong's upmarket hotels. ''The 32-year-old French blonde is said to have asked $3,000 an hour from her customers and was well-dressed when she was arrested inside a Mongkok hotel.''