English is the most widely spoken language on this cosmopolitan little dot in the ocean. That doesn't mean that some knowledge in French couldn't be of help on French St. Martin... But every local resident remotely involved with the tourism industry will be able to assist the visitor in English.
Dutch is the official language of Sint Maarten, used in legal documents and throughout the court system. To confuse matters a little further, the language of the Dutch West Indies with the larger islands to the South of Aruba and Curacao is Papiamento. As St. Maarten was part of the former nation of the Netherlands Antilles, there are strong ties to these islands and Papiamento is the language of many households.
Spanish is used by the large number of Hispanic immigrants, the majority coming from the Dominican Republic, while French-based Creole is used by immigrants from Haiti.
But again: English dominates public life, even politics on both sides of the island. St. Martin is possibly the only French territory where French citizens use English as their first language. Only in recent years has the growing influence of Europeans brought back some French to the island.
English itself is spoken in a variety of dialects: From thick West-Indian accents to the Southern drawl of American expatriates to Oxford English - its all here. Plus the large number of residents, who have to speak English as their second language and struggle to overcome their own specific accents...