Lucky you. Mine changed to Chinese. And I don't speak Chinese. It happened after I added Chinese character input in the Keyboard settings. Just to type one word in Chinese. But at the time 'Dutch' was selected as keyboard input.
When I deleted the Chinese input-modes it went back to English. But it should be unrelated. So I added English as secundary language in the language menu (cause my primary language is Dutch). That did solve it. After that I could use chinese input modes again.
Appearantly Aperture defaults to English if it doesn't find any supported language, but if it does, even in the wrong places it switches to that. So the solution is:
Add English as secundary system language (I guess yours is Swedish). But in fact it's a bug, and Apple should at least ask your permission before it changes the language of an entire program into something you can't read.