If a spaniard says they will complete a task"manana" whywill it rarely happen the next day

This says that when a "spaniard" says that he will do something tomorrow, most likely he/she will not do it.

First, you can not assume that only because someone mother tongue is the Spanish, is most likely that the person is a procastiner, it may be true that the Spanish culture is a little bit more "flexible" than other ones in the subject, but there is no an actual answer to the given question.