Hardest language to learn.

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7. The blog post in the OP makes claims that seem to be ill-defined and unsupported with data...

Here are just a few questions of many that come to mind while perusing the blog post from the OP:

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• How exactly is the difficulty of pronunciation quantified?
  
  One could think in terms of this: http://www.ipachart.com/
  
  
• Exactly how many grammatical genders are present in Polish?
  
  Does Polish more strongly bear remnants of an active-stative typology than other IE languages: http://www.degruyter.com/view/j/lity ?
  
  
• How many cases are present in Finnish? (I suppose "countless" is equivalent to 14 or to 15 in the blog post author's mind.)
  
  This linked author seems to be able to count the cases in Finnish just fine: http://www.cs.tut.fi/~jkorpela/finnish-cases.html
  
  
• Why does the author write "that it out ranks (sic) Polish...." instead of "that it outranks Polish...."?
  
  Perchance care in writing should be one of this blogger's goals: http://dictionary.reference.com/browse/outrank?s=t
  
  

Maybe Polish is very, very difficult to learn.

However, the blog post in the OP fails to present a well-supported case for that position.