I don't see it as a mistranslation Grant, bear in mind that most people in Britain and America in the late 18th or early 19th century would have had no idea what brioche was. Today we can buy it (or a reasonable imitation of it) in supermarkets.
The same problem occurs many times in translation, not only from French to English but between all languages.
The task of the translator is to covery the sense of a set of words rather than the literal translation.
An example in my favourite phrase from Swedish: "det löser sig sa hagan som skit i diskbänken"
This translates as "It dissolves, said the hag who shat in the sink."
Doesn't make a lot of sense, but translated as "It doesn't matter," or "it will go away," we see the hag's words contain some wisdom.
(Note: In Scandinavian and north European Celtic languages hag / hagan means witch rather than simply an ugly old woman.)