More similar to Lepen. In fact they are close on many subjects, Lepen being extreme right and Melenchon being extreme left.
To coin it simply : beyond Le Pen on the right side there is... nothing. No other party. So, technically Le Pen is indeed extreme right. Yet the main reason why to tag Le Pen "extreme right" is its fundamental views : ethnic, xenophobic, anti-semitic influences, islamophobic, rather incompetent regarding public policies. These points will be vehemently disputed by this party and Marine Le Pen worked hard to soften the picture. Yet, fundamentally it is correct to state it is extreme-right : it is conceptually and historically. It is also little known yet well analysed this party actually picked-up many political concept from left movement. Yep : you see it is the other way around, Front National try to look acceptable and credible by diverting ideas from the left. Nice tactic isn't it ?! Classic.
Mélenchon (his movement is currently named "La France Insoumise" or FI or Greek symbol Phi) is a all together different story. Many have a strong interest into shaping this politician as an "extremist" : it would so much easier to discredit him. Yet he is not. At all. That would be as bold as stating Berni Sanders is a dangerous extreme left. Bold and... incorrect. Indeed he severely questions EU and globalisation as also do some... centre party, not only extreme right. That does not make him extreme.
First item : beyond Mélenchon on the left side in France one can find at least 2 groups : NPA (Nouveau Parti Anti-capitalist) and LO (Lute Ouvrière). So, simply put : no Mélenchon is technically NOT extreme left. Second and more important item : these two party (NPA and LO) are anti-republican (they do not believe in the concept of Republic as a collective type of organisation). They promote armed revolution. Their view is very much labour-class centric. These party (NPA & LO) are technically and conceptually really extreme left. On the other hand Mélenchon promotes an "extreme-republic" (his words) view. He calls for pacific drastic changes in the form of a 6th republic. He cares not only for the labour-class but more importantly for the human-class : the one that is being shredded by necro-liberalism and environmental lunatics. He for example explicitly calls for legalisation on long-term-illegal-workers. He is not maniac about Muslims or other religions (He calls for a clear state-church split. He thinks citizen should believe what they want at home and at church. (He also stresses in France more than 50% is agnostic or atheist)). And so on...
So Dude wake up ! You may not like Mélenchon or left or far-left or or extreme-left or socialists or your auntie or you name it : fair enough. Yet, you cannot just go banana and disseminate random idiocies. I also notice that for me to give a little context, it takes... this. While for you to libel it is as easy as a one-liner idiocy. This is classic malevolent rhetorical technique : dump a crap-load and force your opponent to waist energy and time on the clean-up. Did that for you.
Sorry, I don't buy your bullshit.