You can say 'extremely hard' or 'much harder' - but you can't mix the two into 'extremely harder'!
Why? ~ Because 'harder' has a sense of movement - the hardness is increasing whereas 'extremely' suggests that you - reached the limit - reached the extreme - hit the wall - and... stopped.
So that's why they sound wierd or wrong when mixed together - it's the conflict between movement and the movement is over which sounds incongruent.
Both of your sentences are grammatically incorrect by the way, in my view, but you might hear the second one in spoken language.
Correct examples could be:
"I find listening to French much harder than reading French."
"I find listening to French extremely hard, compared to reading French."
In the second example, I added 'compared to' to replace the 'than', so that we could use your 'extremely' without having to use the (incorrect) 'harder'.
As 'harder' is a word that already carries the meaning of comparison (in the 'er' - which means 'getting more so') - without the 'er' we are missing that element, and so need to then say explicitly that comparison is going on.
The reason why 'worryingly harder' works - is because they are both going on - both 'worrying' and 'harder' carry a sense of movement - so there is no conflict there!