One consequence could be that he is feared by others. Fiendfyre is a very dangerous and dark magic, so if Harry is seen controlling it, some people in the wizarding world might view him with suspicion and fear, thinking he is turning to the dark side. Another consequence might be that it drains his magical energy a great deal. Controlling such a powerful and volatile spell could take a toll on his magical reserves, leaving him weakened after using it.
In some fanfictions, Harry might gain the ability to control Fiendfyre through intense magical training. He could be mentored by a powerful wizard who teaches him the dark arts in a controlled way, allowing him to master Fiendfyre. Another possibility is that he inherits some ancient magic that gives him this power. For example, if he discovers a long - lost family heirloom that has the magic to control such dangerous spells.
One interesting aspect could be the power and danger it represents. Fiendfyre is a very powerful and uncontrollable dark magic. In fanfiction, seeing Harry Potter use it might show his desperation or a new level of his magical abilities. Maybe he uses it to fight a particularly strong enemy that can't be defeated by normal means.
It can create a lot of chaos. Since fiendfyre is so powerful and hard to control, it can quickly turn the story into a high - stakes situation. The narrative might shift from a more typical adventure to a race against time to stop the fiendfyre from spreading out of control.
There aren't many major differences. The English version might have some minor tweaks in translation to make it more understandable for English readers.