Just to briefly share my personal experience on the issue:
When I first began attending Liturgy, I think there were only two cradle Orthodox in my Slavic-tradition OCA parish, and they were natively American. There was only English in the services.
Several months ago, we began to receive immigrants into our parish. After a little time, we ended up with several immigrants, including a whole family. Our priest, although a Protestant convert, also is well-studied in Russian language and history (he has an M.A. in Russian Studies!) and can celebrate the Liturgy in Church Slavonic, as well as Greek. Now, his standard policy concerning languages is to incorporate more Slavonic into the Liturgy. Most of the service is still in English, but occasionally a blessing or a litany will be in Slavonic.
My response to this...I LOVE
it! I've been in the Church long enough to know the fixed parts of the Divine Liturgy, and so even if I don't know each word, I know what's happening. I can still follow perfectly, and have even learned a few phrases to know when to cross, bow, etc. To me, it's wonderful because my church counts Russia has its mother church, and so to hear the Liturgy in that language reminds me of the historicity, of the catholicity, of the Church. I hope to attend an entire Liturgy that is done in Slavonic at some point. I would have to feel the exact same way about hearing it in Greek, since they are the mother church of our mother chuch!
And while hearing the liturgy in Romanian or Serbian or Arabic wouldn't have that historical tie to the church which I came into Orthodoxy through, it is still a reminder of the catholicity of the Church.
However, I do understand the concern of a missionary church (such as the American Church is) of trying to draw converts who freak when they hear a foreign language in an already unfamiliar liturgy. It is most likely, at this point in America, not a good idea to hold regular Sunday services in a foreign language. I mean, if the parish is mostly immigrant and wants to stay that way...go ahead, doing that will probably work. But, as a Church that wants to grow, to be mission minded is to do the services in the language of the people.
If there are parishes that desire to have services in other languages, perhaps a good idea would be to hold services on Friday evening and Saturday morning in that language, then have English services on Saturday evening and Sunday morning. Of course, this is not something that many could do. Our poor parish priest would be exhausted, to be sure, and we could not expect him to do so many services so close together! I know I would never ask my priest to do so much work. Another option is maybe to do one weekend cycle of services in a given language. One Saturday/Sunday a month is done in Greek, Slavonic, Romanian, etc. and the others (mostly) in Englush. Of course, there is the issue concerning inquirers who happen to come on those days.
I think most people who come to our church now, although they may be surprised by a Slavonic litany, quickly learn that we aren't some weird group of immigrants. Most of us are simple Southern folks, born-and-bred. This is quite obvious when you meet some of us! I hope that our friendly, American demeanor is enough to undo in possible alarm caused by foreign languages in our services.
Κοινε is not the language of the people.
Κοινη was the language of the people of the Roman Empire at the time of its Christianization. The secular language evolved, the church language did not. Greeks who have attended church for a long time can understand Koine just fine. The same is true with Slavonic in Russia. Those who do not often, or are not Orthodox, will have trouble, though. This is a drawback. Another beauty about this, however, is that when one goes to church, they hear a language that is spoken no where else but in the services. It is another way to set apart the divine from the profane and remind us that we are joining something other-worldly...something timeless.