The Japanese language does not have an "L", and therefore it is spelled out to be Boruto. It was confirmed to be "Bolt" by Kishimoto to showcase that the name holds meaning to Neji's which means "Screw" - Though you don't see us calling him Screw Hyuuga Lol
Even Kishimoto calls the movie "Boruto: Naruto the Movie" not "Bolt: Naruto the Movie". That should satisfy your question.