Unknown (Stated Gold) Coin

could you please provide a diameter?

Hello and welcome.

I cannot tell what language is on it - it does not appear to be Latin, as is usually found on mediaeval coins. I suspect it may be Serbian or Bulgarian.

It's mediaeval in style and script, but looks very modern in terms of production - it's very round, in a way that genuine mediaeval coins never are. Plus, the war this coin seems to inexplicably combine heavy wear (the details of the "face" are almost gone) with sharp details (the boundaries of the letters look extremely sharp) makes me suspect it is a modern fantasy or restrike of some kind, made from a replica of a mediaeval pair of coin dies.

Several mints in Eastern Europe (Prague, Budapest, Sofia etc) often produce restrikes of historic coins for sale to tourists and collectors. "Renaissance fairs" - mediaeval re-enactors - also make coins much like this as a demonstration of the mediaeval arts. As such, I cannot tell you if it really is made of gold, or just a gold-coloured metal. You'd have to get that tested.