Thanks for this. You’re right. I cannot find this window title text anywhere either. You can see it in other non-English sites too here (which doesn’t have qTranslate installed) and here (which does).
It’s good that Yoast fixes it. And I note that you can use the qTranslate shortcodes in there too, which makes it even better.
But this made me wonder: Was Yoast causing it? I just deactivated Yoast and noticed that Page Not Found changed to Page not found. So no, it’s not causing it, but it alters it.
So if you have Yoast (or, I suppose, another SEO plugin) then you can change it there.
But if you don’t have Yoast, it needs intercepting somehow. The only place I can find “Page not found” is in WP’s own general-template.php.
I also checked out the no results page from the search page and I see that the same is happening. I shows Search Results in the title. Again, you can change it in Yoast (where the default is You searched for …), but I can’t figure out where this is being picked up from without Yoast installed.
@nikeo: any ideas? Can this be caught and translated somehow?