Had studied. When talking about your wishes for the the past (things you would like to have done differently) you use the past perfect. When the wish is for now (I wish I knew German better) then use the past simple. So past wish = past perfect Present/future wish = past simple

Studied or had studied are both correct

Studied

Mark Mathews is correct. « To wish » is to want an unreal (hypothetical) situation. Most textbooks explain « to wish » along with type-2 and type-3 conditionals, because they use the same verb forms to describe unreal situations.