It’s not very surprising that Japanese is among the most difficult languages to learn, but more interestingly, English is Category V language for the Japanese. The grammar is similarly painful both ways, but Japanese pronunciation isn’t that difficult for an English speaker. However, English pronunciation is super difficult for a Japanese speaker. And when one looks at the map below, one cannot help but think that it all has to do with the geographical distance, too.
Here’s a map for Europe, also based on FSI data: