About [[MediaWiki:Mw-ui-feature-user-count/en]]
In the first parameter there's "one user is", which means that if there are 21 users, it will show not "21 users are", but "One user is". So I suggest changing this line to "$1 user is" and adding another |1=One user is, if this works, of course. In some languages the translation for 1, 21, 31, 41 ... users would be the same. But now instead of all these numbers it shows only "One user is", which is wrong. So this message should be improved somehow not to confuse the translators that aren't aquainted enough with magic words.
The PLURAL use in this message is perfectly as it should be. I do not see any problem. It there are 21 users it displays "21 users are" as it should. So it has either been fixed, or I do not understand what you want to tell.
In yout translation, you must use the PLURAL parameters of the target language of your translation. If that has a "n=0" case already, use it. Else append the explicit "|0=No users …" at the end.