Hardest Language to Learn?

One of the first questions linguists get asked is "What is the hardest language to learn?" Or, to be quite honest, the question often comes out as "What is the easiest language to learn?" Either way, though, the questions are based on the same premise: that there are languages that are easier/more difficult to learn than others.

My answer is typically the same--it hovers somewhere around "all languages have their own unique difficulties." However, if the person who is doing the asking is actually interested in having a linguistic conversation, I like to bring up the fact that some languages may appear easier or more difficult to learn, depending on their native language. For example, as a gross generalization, if your native language is English, it will be easier for you to learn another Indo-European language like Spanish or German than it will be to learn, say, Mandarin Chinese.

I had never seen someone break languages down into difficulty by the estimated required hours to become proficient in the language until I saw this picture below on 22 Words:

Chart from 22 Words

The entire chart assumes a native English speaker is the one learning a new language and then breaks the target languages into three groups. What I wish could be conveyed by the chart (but the chart would quickly become less of a graphic and more of a document) is what features of the languages were used to determine whether a language is closely related to or significantly different from English, how the number of hours were calculated for how long it takes to reach proficiency, and how proficiency is defined for this chart. I don't quite agree with the groupings--for instance, I have a hard time believing that--for the purposes of this chart--Hindi would be in the same level of ease for an English speaker as Polish.


What do you think? Can we group languages by difficulty of learning according to native language? If so, do you agree with the image above?