Merlin has human appearence , though she manages magic like almost nobody. Is she human or is she supposed to belong to some wizard race?
Merlin is a human from Belialuin.
Belialuin was described as the "capital of the wizards" 3000 years before the events of Seven Deadly Sins. Although Merlin's true name has been shown to use an inhuman pronunciation and alphabet, the latest chapter (Chapter 337) showed her childhood extensively, detailing her motivations and origins.
Unless these facts were misleading or those facts are retconned later by the author, Merlin is fully human.
Most importantly, the Lady of the Lake directly describes Merlin as being fully human, contrasting her with Meliodas and Elizabeth. Her humanity makes her a descendant of Chaos, like all other humans, which directly ties in with her motivations (freeing Chaos from its divine seal). All of these informations can be checked in chapter 337.