ranking the other languages based on how easy it is for us to learn them
These kinds of studies have already been done by the Foreign Service Institute (U.S. CIA training center, among other things). The Institute has ranked languages into four basic categories based on their difficulty and complexity for native English-speakers (albeit American-English exclusively, I bet).
The results are roughly this:
Easiest to learn: Danish, Dutch, French, German, Creole, Italilan, Norwegian, Portuguese, Romanian, Spanish, Swedish, and the like
Next hardest: Indonesian, Malay, Swahili
Next hardest: Bengali, Bulgarian, Burmese, Czech, Greek, Hebrew, Lao, Polish, Russian, Thai, Vietnamese, and the like
Hardest: Arabic, Chinese, Japanese, Korean, and, interestingly enough, American Sign Language
These studies are done based on the average number of study hours spent to achieve specified levels of fluency. However, as any statistic is, they vary widely with individuals.
The important point is that to measure "foreignness" and "easy of mastery" does depend on one's starting point. The rankings would be exactly opposite for native speakers of Chinese and Japanese, for example.
