As with elsewhere in mainland China, standard Mandarin is the main language taught in schools and the main language in the official broadcast media so expect all educated people to be fluent in Mandarin.
The mainstream Minnan (Southern Min 闽南语 ; 闽台泉漳片闽南语 Hokkien-Taiwanese) is the local dialect in Zhangzhou. The local dialect is fairly influential due to the influence of the neighbouring Taiwan that uses Taiwanese Minnan (台灣閩南語/台語) as the second most common spoken language across the Taiwan strait.
The locals in Zhangzhou are proud of their Minnan language despite the central government attempts at promoting Mandarin as the common language. The locals in Zhangzhou speak the local dialect and Mandarin in their daily lives. The locals speak a Zhangzhou accented form of Minnan , which is slightly different from the standard Xiamen dialect accent and the Taiwanese Minnan (Tainan prestige accent). There is also a slight variation in the Zhangzhou Minnan dialect as well. Nevertheless, the locals understand Xiamen dialect or Taiwanese Minnan due to the influence of Taiwanese Minnan language entertainment media. Any attempts to speak mainstream Minnan (闽南话) will be met with encouragement, and may even get you preferential treatment in shops and restaurants. If you are a Mandarin-speaking foreigner, you will be able to communicate with locals and get around as most services have staff who speak Mandarin.
Minnan is not mutually intelligible with Mandarin, Cantonese or any of the other Min dialects except for Teochew (潮汕话) which has a limited mutual intelligibility with it.
If you are a Mandarin-speaking foreigner, you will be able to communicate with locals and get around as most services have staff who speak Mandarin. English is not widely spoken, though staff in higher end hotels will usually be able to communicate in English.
There are buses from Xiamen.
4 Fruit Soup (四果汤; si guo tang) is one of the famous Minnan snacks (闽南小吃), and is not to be missed! A bowl of various fruit, typically more than four, is served in coconut or cow milk. You can get it hot for the winter and iced for the brutally hot summer. 4 fruit soup is truly unique, and will surely be a highlight of any trip to Zhangzhou.