This is not an answer to this question.
The quote does not say when the “French–English version of Plain format” was distributed, but it must have been sometime before the autumn 1989 mentioned.
If you look at the TUGboat list by author, you'll find that Jacques Désarménien published an article in TUGboat issue 5:2, November 1984. The article is titled “How to Run in a French Environment: Hyphenation, Fonts, Typography”.
It describes a French version of Plain (not the combined French–English version described in the question). The short version seems to be that they defined a special font by adding ten accented characters (and guillemets) into non-ASCII positions, and called the resulting font “French Computer Modern” (see Appendix of the above). They defined macros
\fit for roman and italic type in these fonts (alongside
\it which continued to use the regular Computer Modern). Then they generated hyphenation patterns for French.
The article also mentions that further details on the hyphenation are described in a reference [d]:
J. DÉSARMÉNIEN, La division par ordinateur des mots français avec le logiciel TeX, preprint.
This seems to have been published a couple of years later as:
J. Désarménien, La division par ordinateur des mots français : application à TEX, in Technique et Science Informatiques, vol. 5, No 4, 1986 (pp. 251–265).
(going by references here and here and elsewhere). Unfortunately I can't find this article online (and can't read French anyway) so this looks like a dead end for me. But maybe someone else will be able to find it or use the details to think of something.
So it appears (from the quote in the question) that sometime after this 1984 article, Désarménien came up with a combined French-English version, in which, in addition to the French hyphenation described above, English hyphenation continued to be available. Perhaps details are available in the 1986 paper.
(My theory: From the mention of John Hobby in the description, and some hints already present in the 1984 TUGboat article, it may have something to do with fonts. The TUGboat article mentions that an upcoming version of METAFONT would support 256 slots instead of 128, and the idea of putting another “copy” of the alphabet in the upper half (positions 128–255) of 256 available slots. Maybe this is what they did (even though TeX itself only supported 128 characters at the time), and somehow used English hyphenation in one half and French in another.)