This region has its own language, called Teochew or Chiuchow. Teochew is not mutually intelligible with Mandarin or Cantonese, but is partially mutually intelligible with Minnan, spoken across the border in South Fujian. The accent and pronunciation varies slightly between the three main Teochew-speaking cities of Chaozhou, Shantou and Jieyang, but if you are fluent in Teochew, this should not pose too much of a challenge.
The dialect spoken in Shanwei is called the Haifeng dialect which, while closely related to Teochew, is distinct from it, and often described as an intermediate between Minnan and Teochew. There is also a Hakka-speaking minority.
The region is part of Guangdong province and many people speak Cantonese, albeit as a second or third language rather than natively. As with anywhere else in China, standard Mandarin has been the language of education, government and most media for decades, so nearly all educated people are fluent in Mandarin.
English is not widely spoken, though staff at the higher end hotels will be able to speak English.
Teochew cuisine is distinct from the Cantonese cuisine that is prevalent in much of the rest of Guangdong, and is more similar to South Fujian cuisine than to Cantonese cuisine. It is known in particular for its seafood and vegetarian dishes. Braised meats are commonly eaten in Teochew cuisines; a feature which it shares with South Fujian cuisine, but stands in contrast with the popularity of roasted meats in Cantonese cuisine. Other signature Teochew dishes include cold crab (冻蟹), pork jelly (猪脚冻) and fish balls (鱼丸).