sinn er siður í landi hverju

Etymology
From +  the third person singular of  +  +  +  the dative of  +  the neuter dative form of. Literally meaning "its [own] is the custom in each land", "customs are unique for every country" or "each country has its own customs".

Compare the Faroese phrase.

Phrase

 * 1) when in Rome, do as the Romans do