郷
に 入って は
郷
に
従え
ごう に いって
は ごう に
したがえ
Gou ni itte, gou ni shitagae.
This idiomatic expression is oft translated When In Rome, do as the Romans do, however to glean some context and reevaluate its meaning let's look at the kanji involved. The character 郷 (gou/gō) used here means village or district, but the connotation is rural, rustic, down home as in words like kingou (近郷 countryside), zaigou (在郷 rural area, out of town; which incidentally sounds like 罪業 a sin or crime).
So while the meaning to Westerners is very much "When in Rome, do as the Romans" be aware of the less refined implication in a polite situation. A less compatible but more accurate translation is more like When you're back on the farm, do as the farmers. With this in mind, use it with friends after a couple of rounds at an enkai: "when in drunk, do as the drunkards!".
See also:
Japanese Expressions and Kotowaza