If a player loses his footwear while scoring a goal will it be considered a legal goal?

Because it was an accidental, the goal would still be considered legal as there will be no violation.

"If a player loses his footwear accidentally and immediately plays the ball and/or scores a goal, there is no infringement and the goal is awarded because he lost his footwear by accident."

NB: hims056's answer is correct according to the law but I will add the following supporting information.

A lot is said in football about the referee's "interpretation" of the rules in a match. In the interpretation of this law, we should look closely at the term "immediately".

You will very rarely see a referee award a free kick against a player who passes or shoots with an un-booted foot. The player is at a disadvantage by being missing a boot so the generally-seen interpretation is that the player has had his shoe removed by accident.

With regards to the immediacy of the event, you're not likely to see a player running around for 15 minutes without a boot so pretty much any time from the loss of the boot up to the next time there is a break in the play could be deemed (interpreted) as immediate.

To sum up the interpretation as simply as possible; if the player has lost a boot and he hasn't had chance to replace it, then he will not have a foul awarded against him.

Earlier this year, Alessandro Matri scored for Juventus wearing no boot on his right foot.