Pretty terrible. I made an effort to try Bing and ddg a while back, and while it was good enough for searching for English language sites using English words, it fell down heavily when searching in Swedish.

I think the problem isn't just crawling, it's parsing. Google does a lot of clever stuff where it tries to understand the site and handle things like alternate spelling, synonyms and typos. From what I've seen Bing at least is far behind on this front for non-english sites and I don't know if they're even really trying.