Both in Germany and Croatia, that's enforced by the market managers, aka the local government.
Obviously there's exceptions for stuff that's out of season or doesn't even grow in the country - but they do a darn well job at ensuring everyone plays by the rules.
I’m quite envious of that situation, here it’s an anything goes for the provenance of goods and the onus is on the consumer to be ever vigilant in almost all markets and guaranteed to get worse for a generation at least (federal judges being lifetime appointments).