Searching by Finder Tags not working in Spotlight on macOS High Sierra
After searching the internet a little bit, I found a recent post in Apple Discussions explaining the reason for bug. In non-english versions of macOS (at least German and Slovak), searching by

tag:&lt;name_of_tag&gt;


query does not work. Replacing the tag keyword by kMDItemUserTags fixes the problem and properly shows relevant files even from Spotlight:

kMDItemUserTags:&lt;name_of_tag&gt;