Is "Eat what you wish for" natural in English? I have never heard it. Wouldn't "Eat whatever you wish/want" sound more natural?
"Mănâncă" is the 2nd person singular imperative mood of the verb "a mânca" = "to eat." However, I believe that you can also say "Mănânci ce vrei."
I'm not too sure about the difference in nuance. The first one sounds more like a command or invitation, the second one more like "Eat whatever you want."