How often do people say “Mashallah” in your language, and who usually says it?

In BiH it’s commonplace. It is used the most by the Slavic Muslims but the Christians of BiH say it too, regardless of the region. Amongst Slavic Muslims, all age groups will say it. Amongst the Christians, usually older people will say it, but it may “slip out” of a younger persons mouth.

Croatia less so. I think maybe people from Imotski and I had a friend who was from Slavonia who said it, but I can’t speak for the region. In any case, if it is said in some parts of Croatia I’d think it’s only older people.

EDIT: In both cases, it is said ironically aka not seriously, with only a very small amount of Muslims using it seriously