"哪凉快那歇着去" can be translated into English as "Go find a cooler place and rest there" or, in a more idiomatic and slightly humorous/sarcastic way, "Go cool off somewhere else".
The first translation is more literal, conveying the idea of finding a cooler place to rest. The second one captures the slightly dismissive or sarcastic tone of the original Chinese phrase, implying that the person should go away and find a more pleasant environment.