Which do you think would be more important when going to a new place: being able to understand and speak the language like a native, knowing the customs and practices of the place, or understanding the streets like somebody who has lived there for decades?
edit: as somebody described it, it’s language vs customs vs lay of the land
I guess I’ll go against the common answer. I choose “customs”.
Learning customs requires vastly more time than language. Each situation would have its own nuance given the parties involved, the prior events, and the intent you want to communicate. Social class, religion, and history all influence customs. Dress, speech, body language, all are part of customs that are subtle and not necessarily written down to study.
To be able to instantly know all the customs for every situation would be amazing! Language can be learned in isolation and with the instant knowledge of custom language learning itself would be significantly easier.