May I begin by saying that the prose of Cranmer and his associates was written to be spoken and no doubt they tested it out in speech before they sent the copy to the printer. Though it was not identical with any specific form of spoken and written English of the time it was such as to be understandable and usuable by all! And herein was its (as later was recognized) genius and it stood the test of time as the language of prayer/worship for ALL the English speaking peoples from 1550-1950 - a long time. It is still the preferred form of language for a sizeable minority and could be for more if the powers that be would allow or encourage experimental use.
(On Cranmer's prose I heartily encourage members of this Listserve to read the excellent study of the origins of English prose in the book from Cambridge University Press -- Ian Robinson, The Establishment of Modern English Prose in the Reformation & Enlightenment, 1998. There is much valuable material on the BCP etc in here. So many people have half-baked ideas as to what kind of language is that of the BCP and this book is the very best to set the record straight!)
Now some thoughts I glean from a friend concerning what is usually called "traditional" and "contemporary" language for worship. Here what is in mind is both RC and Anglican use of so called contemporary language
The form of English used for common prayer is probably best described from a linguistic standpoint as 'English liturgical idiom.' It isn't really a 'dialect': a dialect covers all the specific or general uses of language for a given group of people who speak that dialect. A form of language used for specific purposes, such as technical language, is referred to by linguists as an idiom. Such idiom is 'contemporary' in so far as it is in current use. I would use the term 'liturgical idiom' to denote the language of Christian worship; though I note that non-Christian English-speakers also use idioms similar to the Christian liturgical idiom for their own common prayer, e.g. the English used to translate the Hebrew prayers in Jewish worship.
What the revisers in the 1960s-1980s and since did was, in fact, was not to use contemporary English but to attempt to invent a new idiom. This was 'artificial' in so far as it was not part of a process of growth or development of an existing idiom, but a linguistic device, based on certain ideological assumptions---demonstrably false assumptions from a scientific linguistic point of view---about what the liturgical idiom ought to be. For one thing, they fail to recognize the distinction between various idioms in a language, and assumed that 'contemporary' English was all one idiom. On the assumption that 'contemporary English'---in fact a collection of other idioms---would be more intelligible, they then attempted to take language from those other idioms (which were tailored for other purposes, some more specific, some more general) and bring them into use for common prayer. The result is an ineffective liturgical idiom, confusing, since the forms it employs already have meanings and, to use a metaphor, overtones, which are at variance with the purpose of common prayer, and in fact less intelligible or, in so far as they are intelligible, false and misleading. The results, confusion about Christian doctrine and a loss of a sense of worship, are evident for all who have eyes to see.
The Rev'd Dr. Peter Toon
Minister of Christ Church, Biddulph Moor,
England & Vice-President and Emissary-at-Large
of The Prayer Book Society of America