It sounds to me as though her English is actually imperfect.
My dad's side of the family is from the Balkans and I'm currently learning the language. I've used online language schools with teachers who speak 'perfect English'. Newsflash - they don't. They're very good indeed, but they don't understand all the nuances of the language the same way that a native speaker would.
That works both ways, of course.
I did a degree in Russian many years ago and had stints in Russian universities as part of my course. I recall one of the lecturers getting very upset at us Brits one day...
The (Russian) conversation starter concerned 'racism abroad'. All was well until someone said in Russian "But of course, there's also racism in Russian..."
Well! The wee Russian wumman became very agitated and insisted that this was not the case. It turned out that at that time "rasizm" had connotations of institutionalised racism akin to apartheid in South Africa at that time.
You will be doing the Bulgarian woman a favour if you explain the negative connotations of "spinster".
I'll add, however that - in my experience - Slavic countries pay lip service to equality but are very patriarchal. As I said to my teacher one time: "Women now get to work and do all the housework and most of the gardening."
I recall my mother being appalled when we visited Dad's relatives for the first time: "The girls are being trained to do all the work; the boys are sitting like little princes."
I'll add that it in the big cities in those countries there's also an expectation for women to look glamorous on top of everything else - women are there to be the object of the male gaze.