Also Germany, Scandinavia and parts of Japan I have found uses this browser. Europe more in percent than America. I am not sure why this is. If any one knows or guesses, I would be curious to find out.

For one thing, it's a Norwegian company, which explains its popularity in Scandinavia, and could mean more focus and expertise on internationalization. Second, some of its features are more suited to Eastern European markets. Computers tend to be older and cheaper, and Opera works well in low-CPU/low-memory environments[1]. Many connections are still on dial-up, and many ISPs charge by the megabyte, and Opera has a number of options for saving bandwidth, like disabling images or using a proxy compression server. These advantages are probably less important nowadays than 5 or 10 years ago, but by now it's an established player, which helps.

[1] On machines with lots of RAM it tends to be a memory hog, but it works perfectly fine in tighter environments. They claim their memory strategy changes depending on local conditions.