You replied in the similar discussion here.

http://www.italki.com/discussion/68560

There was no need to start a new thread with the same topic.

Sentinelese, etc...

I think the harder languages are the less common ones because there are less resources available, somebody who wants to learn a language like Xhosa or Mongolian or something might have a harder time than students of Spanish or French (though I suppose they are difficult in their own way.)

I don't have much experiences with languages yet but I tried learning Nepali and that's incredibly difficult, there's very strange grammar and some tough sounds