You could absolutely forbid english in your own private server, regardless whether you're just playing around or hosting a tourney. Thing is, why would you need to? Wanting to only speak russian isnt the same as trying to keep chat in a language you have the ability to moderate. So while it's absolutely fine to do so, in that situation it'd be rather petty and pointless unless you don't understand english as well as you seem to.

As for the second thing, not every ES is the same. Same basic rules, but they are in charge of their hosting and the way they host and the rules they host with therein. They have the authority in their servers, both as an ES and just as the person running the private server.