This all depends on the makeup of the parish. If all the members are American born, English should be the language, but if there is a good mix of both, then I favor a bilingual Divine Liturgy. We also have to remember, we may have an influx of new immigrants, so they should be able to hear the Liturgy in their native tongue. In our UOCUSA parish we use about 50/50. In fact, there are some parts that both the priest and I do in both languages, as this will help both groups to learn a new language.
It also depends on what you want
the makeup of the parish to be. If you want the parish to focus on its ministry to new immigrants (a necessary ministry, I would agree) then the use of that language is appropriate. However, if you want the parish to focus on its ministry to the children and grandchildren of those immigrants - and to proclaim the Gospel to the community at large - then the use of English ought to be your choice. Please understand that I do not see the two as being mutually exclusive. How that may play out in a parish will differ depending on local circumstances. For example, you may want to have most services in English with a few in the other language; or occasionally instead of, or perhaps in addition to English. Or you may have bilingual services. But what do you mean by that? Some parts in English only, some in other language only? Or by repeating most things? And if the former, will you switch the two parts on a regular basis so no one is left out?
These are just questions. I have the luxury of being in a mission parish that is English-speaking with only an occasional nod to Arabic, and even less to Greek. In fact, those who speak those languages are insistent that we use English. I guess that's why we feel OK with an occasional use of their language, which I know they appreciate though not likely to admit it
I might challenge you though on your statement that a new language will be learned in our services. Most people will simply tune out the other language. They may pick up the occasional phrase, or may learn a few brief hymns and prayers, but that's about it. Conversational ability won't develop. Besides, from what I read in this forum, liturgical Greek, certainly Old Slavonic, perhaps others, have little practical use outside the liturgical services. How many congregations speak Old Slavonic at coffee hour or in the business meetings? What I read once somewhere - probably on this forum - was that the language used in your parish council meetings should be the language used in the Divine Liturgy.