I think you mean 'Genie' rather than 'Genius'.  A genie is a magical being that grants wishes. A genius is a very intelligent person.

I suppose that 'contentment' would be the best thing to ask for in your one and only wish. A person who is content doesn't need to wish for anything else.

If I had a genie that can fulfil one single wish, I would ask for the thing I desire most....

And the thing I desire most is wisdom.

With wisdom, you can corry anything out.

To be fluent in another language besides English ......

Francesca,

I'd offered my help:)

Genius is also one of those spirits from Roman mythology (genio). I mean, it is still one of the meanings of the word in English.
Génie is the same word, but in French, it was once used to translate jinn to French.