What's The Most Difficult Language For English Speakers To Learn?

Once you start looking for them, there are lots of places on the internet that talk about the relative difficulty of learning to speak different languages.  This infographic (courtesy of Voxy.com) sums up what many of these sites are saying:

While I like the infographic, it is impossible to ignore some of the more robust efforts to categorize languages by difficulty. For example, anyone who has been to the Defense Language Institute or the Foreign Service Institute knows that the US government has its own scale that it uses to categorize languages by difficulty.

Even more interesting is the fact that the relative difficulty of the language may or may not correlate with how much extra pay you will receive for speaking a particular language.  For example, compare how difficult it is to learn French with how much extra the Army will pay you each month if you speak French on Page 20 of Army Regulation 11-6...