When going back to the forest Luna retrieves Giuseppe's backpack and finds a figurine of Son Goku with blue hair. Then Luna and Loredana dye their hair blue like the toy.
Son Goku appears in Dragon Ball, Dragon Ball Z, Dragon Ball Kai, Dragon Ball GT, Dragon Ball Super.
The movie takes place in the 90's and Son Goku did not achieve the blue hair transformation yet. Blue hair is a new transformation that appeared with Dragon Ball Super which has been released only in 2015. They should have referred to Dragon Ball Z and a yellow hair transformation (the only one known in the 90's for Sayans like Son Goku).