EDIT: When Le Pen and FN win the next election, they will by definition no longer be extremist, having demonstrated they represent a majority/plurality of the population rather than an extreme fringe.
Sorry, the definition don't depend on the majorities, depend on facts, positions, statements. They are the garbage of Europe, populism, nationalism, loud voice and no brain. They can do or say wherever to get some attention or votes.
Just like you.
Actually yes, the definition does depend on statistical metrics. EG, heliocentricism was once considered extremist, but now it is not.
Here, let me help you learn to dictionary.
If most/enough people support FN (or ideas that aren't very far away), by definition it is not extremism.
I know you Marxists believe in socially constructed reality, so this shouldn't be a problem for you to understand.
Unless of course you only believe in definitions of words which suit your rigidly predefined ideological categorization.