Paco, I'm not sure I understand what specific problem you are having here. You do realise that in hypothetical sentences we use the present simple after 'could'/'would' when referring to an action or happening, whereas, we use the non-past when referring to a state, don't you?
I wish I could help her. (action/happening)
I wish you would tell her. (action/happening)
I wish I wasn't Japanese. (state)
I wish you lived here. (state)
I wish you could tell [action/happening] her I was [state] Japanese.
In this last sentence, though, it might technically be 'were' instead of 'was'.
Does that help, Paco?