Do you know if any studies were made to classify the difficulty to learn a particular language depending on learner's native language?
There are a lot of discussions about what is the easiest or the hardest language to learn, but they are mostly made of assumptions that other languages' users have the same difficulties with the same things, which is obviously not true. The Russian grammar may be difficult for English-speaker, but it is easy and intuitive for any user of Slavic language (which, at the same time, could find English grammar very complicated).
It would be interesting to have a cross-language study, which would classify, in the 2-dimensional table, how difficult is language A for native speakers of B, C etc.., language B for A, C, D etc.. etc... Such studies could be even use, for example, to model how the languages became popular or die. Is the English so popular nowadays because it is relatively easy to learn for the most of the people on Earth, or just because the political and military dominance of Occident over Orient?