Triglossia or a language of record and two vernaculars? -- some brief notes on the 11th-12th centuries.

Perhaps the most important analysis is now that by Ian Short, 'Patrons and polyglots: French literature in twelfth-century England' in M. Chibnall, ed., Anglo-Norman Studies. Proceedings of the Battle Conference XIV (1992), esp. pp. 246-7, which suggests that the insular language did not go 'underground' in the century after the Conquest. England was genuinely bilingual in vernaculars and trilingual if the formal language of record, Latin, is included, although the different languages were used for different purposes. By the middle of the twelfth century, Anglo-Normans had an active command of Middle English, which had become, by the end of the twelfth century, their first language. From the 1160s, vernacular French had been declining and by 1180 formal, grammatical French had become a second, acquired language. Middle English as a literary language flourished in the late 12th and early 13th centuries, especially in the west Midlands. La3amon's Brut, was composed by the priest at Arley on the R. Severn in the 1190s or shortly afterwards. The Ormulum, written by Orm, an Augustinian canon in c. 1170, consisted of ME homilies on the life of Christ. Some of the texts in the Katherine Group, written in ME language AB, have been associated with Wigmore Priory in Herefordshire.
ME continued to flourish as a literary language or code. Different languages (codes), however, were used for different purposes (registers): Latin was the formal written language; French a less formal written language and thus considered more ephemeral than Latin in written form, as well as a spoken vernacular (but not necessarily with formal instruction in grammar and syntax); and ME developed as a literary language as well as the informal spoken language of all social groups. The most important changes occurred in French and ME. French became an informal language, not the language of Paris. Whilst Old English, by being a language of record, particularly in writs, had been maintained by a standard (West Saxon 'Chancery' standard), ME, without that standard as a language of record, changed. First, in the middle of the 12th century, different regional dialects developed since the standard was removed, including its literary expression, and secondly it lost some of its syntax and grammar through the levelling of inflections -- losing some of the declension of nouns.
There are some MSS which are triglossia, such as the Eadwin Psalter from Canterbury. The homilectic pontificals of Osmund, the first Norman bishop of Salisbury, are heteroglossic, since he produced homilies in the English vernacular. Later texts which are heteroglossia including the English vernacular, might be either macaronic or code-switching, for whatever purpose. Traditionally, Middle is assumed from c.1066, but there is a strong case for arguing that the influences which altered OE to ME, required time to become effective -- chenges in metre, orthography, levellings of inflection, loanwords, dialect formation etc.
LATIN Latin continued to be the language of record until it was abolished by an Act of 1731 which became effective on 25 March 1733, although it had been temporarily abolished during the Interregnum from 1653 under an Act of 1651, but restored in 1660.
FRENCH By the early 13th century, formal and grammatical French had been reduced to a second language, a tongue of acquisition, in England. The primer of Walter de Bibbesworth (1250x1260), produced for Lady Denise de Montchesney, assumed that knightly families spoke English and some French, but would want to learn a more syntactical French. Conversely, French developed as a lesser language of record, in less formal and more private documents. A charter of c. 1140, preserved as a copy in a cartulary of the Knights Hospitallers, was unusually written in French. The first extant administrative document in Anglo-Norman is a draft reply to the Inquest of Sheriffs in 1170, but it is assumed to have been a cursive draft engrossed into Latin. An explanatory royal letter about Magna Carta, addressed to the sheriff of Hampshire, was also composed in French. During the Baronial Revolt of 1258-67, letters patent of importance were issued in both French and Latin. The most important developments occurred from the late 13th century as informal royal correspondence was increasingly produced in French. Consequently, new forms of document, authenticated by the newer and lesser seals (privy and secret seals) were written in French, whilst established forms of document under the great seal continued in Latin. In the 14th century, a proportion of private documents, especially letters, but including charters, was also written in French.
These sessions, however, do not address documents written in French.
MIDDLE ENGLISH Before 1066, diplomas had been composed in Latin, but with charter boundaries in the vernacular (Old English), but writs had been produced in the vernacular. The vernacular persisted in use in formal documents until the 1070s, particularly in writs. One reason may have been that the personnel producing the documents before then may have been of insular origin and were only later replaced by Normans.
English persisted as a literary language, but not as a language of record. From the late 14th century, however, Middle English began to recover its status as a language of documents. The earliest extant deed in ME survives from c.1376. The volume of ME documents increased from 1420x1440; for example, from 1422, the admininstrative records of the gild of London butchers were kept in ME. Many title deeds were produced in the vernacular from the 15th century.