Though I don't know anything about which language has the largest vocabulary (this seems like a fool's errand), I feel compelled to point out that all languages are languages of loan words to a greater or lesser degree, and that more loan words do not necessarily lead to a larger overall vocabulary (in fact, the fight against loan words in many language planning/policing situations is often tied to a fear that they will replace perfectly good native words, because that does happen).
Anyway...if Albuquerque's small church is anything to go by, the Coptic church has largely become a church of the vernacular, at least in the USA (I have read that in Europe there are more problems in this area). A typical liturgy here at St. Bishoy C.O.C. will be 75-80% English, but usually repeated in Coptic and Arabic (for the people's responses and some of the litanies, anyway). And at all times there are trilingual translations available, as you have to be able to switch between the three quite quickly and anticipate (from the language being used by the deacons at a given point) which will be used in the response. It takes some getting used to, but it also helps to internalize the Coptic parts and for me the Arabic parts as well. If we do any kind of paraliturgical hymns (e.g., Tasbeha), those are more reliably in Arabic, but even then there is generally some English used, and the translations are available.
I like this system. It apparently works well as we have had an influx lately of non-Egyptians coming to visit (hey, when you're a church of ~40 people, getting 7 visitors over the past two weeks counts as an influx), and none of them seemed put off by it. The only time I have personally struggled with the language was when I had to try to sing "Efrahi ya Mariam" from a photocopy of very small, hand-printed Arabic (with no diacritics, of course) with my Ethiopian friend trying to follow me, since he knew no Arabic. What a disaster that was.
The poetic quality of the English translations (particularly in the Agpeya) is sometimes lower than I think possible, but never obscures the Orthodox teachings. It is at least much better than the translations of HH's books.
I do wonder, though, if perhaps the British Orthodox Church (under the Coptic Orthodox Patriarchate) might produce a better translation into English, being mostly native Britons and hence not having to rely on Egyptian bilinguals as seems to be the case elsewhere.