I was in Bulgaria over the weekend, and had some problems using the card. I’d say acceptance was about 50/50 with some retailers and ATMs taking the card and some terminals rejecting it as an invalid card. My hotel, for example, took the card fine but the Sofia Metro ticket machines didn’t.
I found that those ATMs where it would work recognised it as needing English language straight away, while if it asked me to select a language it rejected the card. Postbank and Банка ДСК (Banka DSK) ATMs were fine, but UniCredit BulBank ATMs weren’t.