I applaud your perseverance through this difficulty. I wonder if you are the Neville from Australia I've been in contact with before.
I am very fortunate to belong to a congregation under Fr Athanasius Iskander here in Canada with ample English in the liturgy. Our Liturgies include Greek, Coptic, Arabic and English with the primary focus on English. We do however have additional liturgies performed throughout the week that are primarily Arabic for those who desire to pray in their mother tongue. The idea of a pure Coptic liturgy is strange to me because very few Copts speak fluent Coptic and even fewer speak it for general conversation.
We are also fortunate here in Canada to have a Coptic mission church under Fr Pishoy Salama which is completely English and is geared toward those of ethnicities other than Coptic.
I have been witness to the "visiting of well-trained deacons" spoiling the English content. There is a sort of unspoken understanding that whatever language the priest uses, the deacon replies should be in the same language, but there are those who insist on replying in Coptic, which appears to me to be somewhat disrespectful.
Praying in the local language does not impose a deterioration of the culture, but cultures do morph over time. Third generation Canadian Copts may not identify with a culture they have never lived in, and so are likely to feel alienated if the church they attend feels foreign to them; so why not change cultural aspect of a congregation to meet the needs of that congregation. When the BOC joined the Coptic church, they were not expected to start praying the liturgy in Coptic or Arabic.
I will pray for those in Australia who desire a culturally relevant parish so that the Father might be glorified in all tongues and that those who speak English might take the blessing that comes from prayer.
Glory to God