I had a similar question and posted it here How to tag shops specialising in products from a certain country?
I understand that the consensus is that such shops should be tagged like they would be tagged in their home place (an Italian food store should be tagged like it would be if it was in Italy, for instance) and additionally with cuisine=* if it is mainly selling (processed) food and origin=* if it is mainly non-food. Is this something we could document in the wiki?