Kelantan/Kota Bharu is generally safe, and people are generally friendly.
respect
More than anywhere else in Malaysia, in Kelantan it's important to respect local mores, especially if you head out into the countryside. It is not necessary or expected for non-Muslim women to wear a headscarf though it would certainly meet with approval, but both men and women should wear clothing that covers all or at least most of their legs no short shorts, and women should wear blouses or dresses which cover their torso. Long sleeves are preferable for women, and outfits which show the shoulders or midriff should be avoided. The watchword is modesty. Tourists dressed in shorts and skimpy tops are really out of place in this state.Regardless of how modestly you dress, if you're white, people may stare at you. If that happens, just smile.
talk
The Kelantanese dialect of Malay is famously incomprehensible to outsiders, so much so that Kelantanese and non-Kelantanese alike jokingly refer to it as German pronounced gher-man with a hard G, but yes, it means German. Nevertheless, all Kelantanese learn standard Malay in school, so speakers of standard Malay should not have any problems communicating.
The Orang Siam also speak a dialect of Thai called Tak Bai, but this is also quite different from standard Thai or even the forms generally spoken in southern Thailand.
As for second languages, some people speak English, but do not expect everyone to be able to. And while no-one will expect you to speak Bahasa Kelantan Kelantan dialect, some knowledge of standard Malay would be very useful.