What is the most difficult language to learn -- and why?

There are numerous rankings of "difficult" (and "easy") languages to learn (note that we are talking about second-language learning here, not acquiring one's native tongue). Some such rankings are unofficial, like the Accreditedonlinecolleges.com ranking or the ranking at MyLanguages.org; others are official, for example the classification by DLI based on the number of hours needed to achieve a certain level.

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