I only use it in English but I've found the results to be equal to or better than Google most of the time. I rarely end up checking things in Google after comparing the results for the first few weeks I first started using Kagi, the times I have recently Google has been just as bad (e.g. no results found, or nothing useful) or worse (e.g. a bunch of SEO crap and advertising and whatever other nonsense Google decides to spit up).

My understanding is that the up or down rating pushes results from that domain up or down in your personalized search results for all searches, not just the current search. I flag down things occasionally because they are obvious shit or don't interest me but haven't used the feature much otherwise. I think it could be one of the reasons I find Kagi better than Google though, there's so much crap that just doesn't end up in my results now because I down ranked already. I'm not completely certain that is really how the feature works though, I'd encourage you to contact Kagi support for a definitive answer.