My priest has said that when we pray, we pray corporately, which, to him, means we use the exact same language. Now, most of my prayers are in Greek (it's what I'm used to) and he knows that I pray many things (except for things that are sung) like the Creed, the Communion Prayer of St. John Chrysostom and the Lord's prayer in Greek. He's told me to stop essentially saying that I risk my salvation when I am not praying corporately. My response is that I am praying and the language issue is irrelevant. He's a priest who believes that everything should be in English.
If this is really the meaning of corporate prayer, then I think I haven't properly prayed ever in my life.
"Corporate prayer", to me, is simply the prayer of the Church (i.e., liturgical prayer). Each of us has our own personal prayer life, but when we gather together as the Church, we pray with one voice as one body, and so we use the Church's prayer. I would hesitate to absolutize a definition beyond that.
If, for example, an English speaker attends a service where 70% is in an unfamiliar language, and they join in as many of those prayers as they can by saying them to themselves quietly in English, I think that person is participating in corporate prayer. If he knows the language well enough to join in the singing/chanting, then you can argue that it's better to pray aloud in the foreign language than to pray to yourself in English, but still I think it's corporate prayer. That works, of course, in the case of those for whom English isn't a familiar language in churches that use English. Now, if you're an English speaker in an English church who uses Slavonic at home "just because", and insists on doing so at church, I think it's still corporate prayer because it's still the same service; however, I would want to know how much of this is habit versus, for example, a Slavonic fetish.
I think if "corporate prayer" is limited to using the same language and same texts at the same time by all participants in a particular service, then translation issues and language abilities are only part of the problem. What do you do with traditions that use choral music? Four-part harmony sounds nice, but it's still four parts, it's not one voice the way chant is. Are the Russians doing it wrong? Is their salvation also in jeopardy? What about speed? I've been to churches where things are dragged out beyond recognition, and others where the Our Father is prayed so fast, with all the attendant sloppiness, that only a God could understand it. If they're at "Deliver us from the evil one" when I'm at "Give us this day our daily bread", who's not praying corporately--me for being too slow or the reader for not properly leading?
And, to me, such observations only touch on the letter of the issue. What about the spirit? If I go to a church where the Liturgy is in Georgian, and I can't make heads or tails of anything, so all I do is follow what I can of the service and say the Jesus Prayer or something, is my salvation in jeopardy? Or is that more "corporate prayer" than simply gawking for two hours? Maybe just showing up with a good intention is already "corporate prayer" that gives your own weak efforts that much more strength.
If your priest really has the attitude you say he does, then I respectfully disagree with him. But I wonder if there's more going on in the background. Maybe he's had some bad experiences with a more "lenient" approach, or maybe people have given him trouble in the past over certain things. Or maybe he just has his own ideas, who knows.