This is sadly based on the french translation and not the japanese one (And I can't find japanese or italian raws to see if they use similar terms). In this chapter, she uses "mon ami" (my friend), which I translated as "dear friend". (Other chapters also use "cher ami", which is literally dear friend). Just like in English, "friend" was an old-fashioned euphemism for lover in formal or literary settings, so it is used here to connotate a high-class, kinda stuck-up setting where displays of love were relatively unacceptable
I could have translated it as "dear", or "my dear", but IMO it was important the unnatural, stuck-upped-ness, "no one talks like this anymore" formal connation of "friend". Notice how, in the previous story, Okane kept calling her husband "love" because they lived much less formal, stuck-up lives. As for the next chapter, pay attention to how they stop using the phrase when they're in the passion of the moment
TLDR: I don't actually know if japanese people called themselves that back then, but people in formal english or french settings sometimes did so I went with it just like the french translation did