When in Rome, do as the Romans do
The Spanish equivalent of this idiom is Donde fueres, haz lo que vieres “Wherever you go/are, do whatever you see”, which isn’t as expressive, but has a couple of verbs in the future subjunctive (a tense most Spanish-speaking people don’t even know about).
The English idiom, literally translated into Latin and Spanish here, actually originates in Latin itself, in Saint Ambrose, whose Latin version is classier than ours:
si fueris Romae, Romano vivito more
That is, “if you are in Rome, live in the Roman way”. It continues like this: si fueris alibi, vivito sicut ibi “if you are elsewhere, live as they do there”. There’s plenty to comment about, but we’ll restrict ourselves to comment about the future imperative vivito. It’s also interesting the adverb alibi (< alium + ubi) “elsewhere”, which the English language used for “alibi” (≈ “excuse”), that is, “evidence that proves that a person was in another place at the time of a crime and so could not have committed it” (source).
In our translation, we begin with a temporal subordinate clause with cum, which would have its verb in the subjunctive —not the indicative, as temporal cum usually does—, since this temporal clause is very close to a conditional (as Ambrose’s original is, in fact), though we have ommitted it anyway because of the English idiom. We also have a locative, Romae, which is the name of a city of the first declension.
Then we have the comparative subordinate clause with ut and the verb —this time— in the indicative. Finally, an “irregular” present imperative ending in a consonant, fac.