userSettings resource doesn't work properly

I’m trying to get keyUiLocale property for a particular user via API. I tried/api/userSettings/keyUiLocale?user=someUsername but it doesn’t return proper results. I guess the problem is here:,credentials.getUser() doesn’t return a user with requested username but the owner of credentials object.

Is there some other way to fetch keyUiLocale of a particular user (I need to get keyUiLocale of any user not only current)?