Vlad I think that the short-term solution will be to use another search engine like Google.
For example, "一" (number 1) search results in X includes chinese and japanese.
https://twitter.com/search?q=%E4%B8%80&f=live
However, X support lang:<lang>
and I add lang:ja
https://twitter.com/search?q=%E4%B8%80%20lang%3Aja&src=typed_query&f=live
If there is a way to specify the language of the search result in Kagi, I think I will use it.
I think that the language that users want is different for each user, so it is quite difficult to automatically determine.
For example, in my case, if the search term is English and Japanese, I want to prioritize English, but other users may not. However, I think that there are very few users who want both results to come out when Japanese and Chinese are mixed(This is because I have the experience that there are very few people who can understand both Japanese and Chinese).
Probably, priority-based ranking is necessary for each user rather than on or off if you try to automate it.