I'm okay with a few non-English words here and there used as greetings and farewells; perhaps even slang when taking a jab at someone's character. Things like that can be a little fun, even if I may not understand a word or two here and there. But at the end of the day, Discovery is an English server.
Even when I blurb Kusarian words on my Freelancer, he *always* gives a translation, so people will know what he just said. If I played a Coalition character, I'd do the same thing (If Russian words even really translated well via an English keyboard).
A word or two is okay, in some cases even a few word phrase. But no full sentences without the poster translating for the reader.
Going all out and speaking Russian only is a bit overboard, but using more words from the language would be kind of cool. Back when I played Kusari actively, there were a few players that used a lot of non-generic Japanese words, sometimes going as far as to make half the sentence in Japanese, but they would always leave a translation in brackets afterwards to avoid confusion. Could work that way if you're looking for more immersion I guess.
* Nodoka Annexes the Discovery Forums, Omicron Delta and Kusari
Anywho, I think everyone's got the general idea - Multiple languages when used fully and not in passing is a bad idea. It leads to confusion when not properly translated and is a hassle.