This is great news for EU and Russian servers, it will result in reduction in queue times, and a larger pool of players to draw from. But for both sides, there's quite the language barrier.
If you are not familiar with Russian, they use a different alphabet, called Cyrillic. While many letters look similar, many do not, and they are often pronounced differently. What Takralus is talking about in this context is that non-Russian EU game clients couldn't display Cyrillic text, so instead of seeing Питер (Peter), they'd see ?????.
The issue, which many players are responding to in Takralus' original thread, is that most EU players can't read Cyrillic. Питер is no more readable to your average European than ????? is. And while the Roman alphabet, the one we use, is far more pervasive than Cyrillic, it's quite possible that Russian speakers, or speakers of other Cyrillic languages, could suffer the same problems with our names.
But why is this even a problem? Names are important. Names allow you, when facing two opponents of the same class, to say "Kill Davethewarlock, CC Bobthewarlock". Yes, you could try to say "Uh, CC Antep? Nntep?" but it's another obstacle.
Previously, the battlegroups have been separated, and while I can't speak for Russian servers, the EU ones have been just fine. It'd be interesting to see what the reasoning behind this change is, as the forums don't seem that happy about it.
Nonetheless, we should all try to accept new players in our midst. Maybe this will be an exercise in education, and serious EU PvPers can learn not a new language, but a new alphabet.