When I search the German word "Nacht" in CC in Windows 10 I get 3 Pictures as result. When I do the same in ios I get over 100 Pictures as result. When I search the english word "night" the results are the same under ios or Windows 10.
If searching for 'night' gives the correct result, then it can't be a synching problem. I vaguely remember that Lightroom CC for the desktop does not yet support search in other languages than English (which would indeed explain what you are seeing), but I'm not sure.