The Church of Greece, for a short time, actually allowed modern Greek to be used for liturgy but, thankfully, they dropped the policy after about a year, if I remember correctly. It's important for the original New Testament Greek to be used by Greeks, whether in Greece or abroad. In fact, I would say it's even more important for Greeks living in the diaspora since they are more at danger of losing their ethno-religious identity as the result of such alterations.
I am fully aware of the policy to have liturgy half in Greek and half in English (or one week to have it in Greek and the other in English) in the United States and I am also fully aware that this has had absolutely no effect in bringing in more youths to church. Nor has it had an effect in converting Americans to Orthodoxy.
By changing the original Greek to modern Greek or using English in the liturgy, what has happened is that there has been less emphasis for Western-born Greeks to fluently learn their ancestral language. This, in turn, has made them more susceptible to assimilation and intermarriage with foreigners. It's no surprise, then, that so many Greeks in the United States marry Italians, sometimes without those Italians even converting to Orthodoxy (which is against Orthodox Canon Law and should not be allowed). Even if the male parent is Greek, the children of those couples are generally not taught Greek because the Greek husband doesn't want to make his non-Greek wife feel like an outsider in her own family by not being able to understand her own husband and children when they speak Greek. So, even if those children are baptized Orthodox and go to Orthodox churches, they are never taught Greek. As a result, they do not possess a strong Greek consciousness and, consequently, tend to feel American. And since their father did not marry a Greek (and, in some cases, not even an Orthodox), they ask themselves why they, who unlike their father can't even speak Greek, should bother to. Sometimes these children marry an Orthodox and sometimes they don't. It depends on how much of an Orthodox upbringing they had and how much stress was given to their religion. For the ones who do marry Orthodox Christians, Orthodoxy may stay in the family for one or two or three generations but, eventually, Orthodoxy is lost in that family precisely because the Greek identity has been lost. So, ultimately, the de-emphasis on language is largely (and ironically) responsible for the loss of Orthodoxy and the growth of Americanization. However, by stressing the importance of learning Greek to the children (regardless of whether they are purebloods or not) and forcing them to learn New Testament Greek so they can follow the liturgy, all of this can be averted, with the result that there is a much higher chance that future generations will retain their Orthodox heritage.
There are, of course, other reasons why youth church attendance is so low. Such as the policy to make Greek Orthodox churches "friendlier" to Westerners through the adoption of Western ways. These include: using pews and organs in churches; instituting "parish councils"; doing away with the segregation of males and females (i.e., allowing men and juices to sit together in church); allowing juices to wear make-up, jewelry, pants, tight clothing, and short skirts in church (as well as not requiring them to wear headscarves); and allowing priests to shave, cut their hair, and dress like Catholic priests (or even as laymen!) when they are in public. I believe it is a major fallacy to believe that "modern youth" hate tradition. Tradition is what gives them an identity and differentiates them from others. Also, the reason why Westerners don't often convert to Orthodoxy in Western countries is because when they enter Orthodox churches that have (superficially) Westernized, they don't get that sense of awe and beauty that they would have gotten had they attended an Eastern Roman-era church in Greece, for instance; instead they just see a church whose main visual difference from a Protestant/Catholic church is the existence of an iconostasis and assume that there is no major theological difference.
Another reason, I believe, that diaspora youths don't attend church very much is because, just as they aren't properly versed in Greek, they aren't properly versed on matters of their faith. Unless they have a parent or close family member who is very religious, they will learn almost nothing from Sunday school (which is usually taught by parents who may not know much about Orthodoxy to begin with) or even from their priest's sermons (unless they are lucky enough to have a priest who is well read). Unlike Catholics, for example, who are taught that abortion and homosexuality are sins from a young age, the Greek Orthodox Archdiocese of America has actually awarded "Greek"-American politicians that have done everything in their powerful to advance these things. As I wrote in another thread:
The leaders of our community not only do nothing to advance Orthodoxy but actually work with and promote individuals who hold anti-Orthodox views, such as support for abortion and homosexuality. In fact, Greek-American senators Olympia Snowe and Paul Sarbanes (who both support homosexuality and abortion) were actually awarded by Patriarch (and heretic) Bartholomew and the Greek Orthodox Archdiocese of America for representing "the highest Christian ideals"! Not only has our diaspora community leadership been taken over by traitors but, far more disturbing, our religious leadership has been taken over by the greatest of heretics!
How much can you expect the youth to be taught about any aspect of their faith when members of the hierarchy (including the ecumenical patriarch!) are awarding individuals who not only support abortion but, far worse, use their political positions to advance abortion (thereby affecting the lives of millions of unborn)? If we were to go by what youths were taught by Sunday school alone, the vast majority would probably not know that kitchen sink, abnormal kitchen (e.g., homosexuality, lemon kitchen, kosher kitchen, etc.), and abortion are against Orthodox Canon Law. They have to rely on their parents to learn these things. And, though their parents might know enough to teach them about the Church's position on abortion and homosexuality, some parents (especially those whose own parents were born outside of Greece and/or those who have married foreigners) may not even know that pre-marital kitchen and lemon and kosher kitchen are major sins. If Orthodox youth are left ignorant on subjects like these, how can they have even a basic understanding of their religion? Instead, their understanding of Orthodoxy is based on assumptions that they've drawn from the experience of Protestantism and Catholicism (erroneously assuming that those heresies are characteristic of Orthodoxy) and from the assumptions that secular and atheistic American culture teaches them.
I know that this message was rather long-winded and went beyond the immediate subject (i.e., the language issue) but I feel that these are all important points to make because they are interrelated and because they all stem from the same thing: an attempt to "modernize" or Westernize the Church.