Both Montenegro and Kosovo have unilaterally (?) decided to use euros as their official currencies. I think that most of their capital exists only as records in bank registers, so there is not much problem with that, but what about physical notes and coins that are in circulation? Where do they get them from?
Countries can get foreign currency from tourists, exports or intergovernmental aid programs. (And several other ways, but these should suffice here.)
Both countries used the German mark before the creation of the Euro. After the Mark was replaced by the Euro, they had no choice but to adopt the Euro. So it was not unilateral. Therefore, the Euros in circulation in these countries are "German" Euros as far as I understand the situation. The coins initially introduced there and most commonly found today certainly seem to be the German versions of the Euro coins.