If I can do the former, I can learn the latter.

Obviously the language and people, because even if you are blind you could just make friends to show you around

Yeah, language is a no brainer. If I know the language, I can find my way around.

You can navigate anywhere: Time and space operate by the same rules pretty consistently, and even when you can't do orienteering, there's always map and compass or GPS.

Customs are easy enough to look up and in many places, they're not even that important to know: You can probably get by without knowing all the customs and if any are really important, it'll probably apparent.

Languages are different all over the place and much harder to pick up: I am only really fluent in 1 language, can sort of understand a couple others, know tiny bits of others, but most of them I do not understand and there are hundreds of them all over the planet. On top of that, knowing the language would also help you ask for directions, find out about local customs, etc.

Language for sure!

Language is a more universal and practical thing in many situations.

Language by a country mile

Language. I would always choose language. That can unlock the other two.

This isnt even a question lol

Personally: etiquette.

Maps and dictionaries can mostly solve the other two problems, but if you come out as rude or do weird stuff, nobody is going to tell you and you could offend a bunch of people for nothing.

Language, easily.

Google maps already lets me navigate almost like a local.

A quick search on the most important customs would cover that fairly quickly, plus a lot is likely to be overlooked because I'm a foreigner.

But language, it takes years to learn a language properly, and even then it's not native level. Knowing the customs and streets is pointless if you can't communicate, and google translate is not going to be that much help in a real conversation.

language 100%.

I like wandering, worst case I get lost, and then I find my way again.

Hell of a lot easier when I can ask for directions.

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.

100% language and customs. I’m pretty good at navigating once I’ve spent a little time in a new place.

I love the idea of the language, but for a dumb reason. Say you go to Korea, have fun, make new friends, and invite them to visit you. Unfortunately, you no longer speak Korean after leaving the country, so you suddenly lose the ability to communicate mid-flight, and you’re this person’s only connection to wherever you’re landing.

I miss read the question at first glance:
Language vs customs vs lay of the land.
Now in that case it's not as easy choice.

Language. I've managed to learn the rest pretty easy everywhere I go but I do not handle language well.

Navigation, easily. I can learn a few basic phrases of a language, and odds are someone will speak English in the places I’m going. I can ask English-speaking people what the essential customs and etiquette are.

But if I don’t know where I’m going, I can’t be confident in what I’m doing. If I’m in a new place and have to follow a narrow set of directions to get to specific places, I’m confined to those narrow spaces. Knowing where I am and what’s around me opens up the world with me driving the experience instead of relying on the knowledge of others.

In both of the cities I’ve lived in, I hear a place mentioned and I instantly can point in its direction, estimate the distance, and calculate a good route to get close to there. At the music festival I’ve attended several times with a large venue I am the guy that people stop and ask for directions because I exude confidence in where I am. To be able to carry that knowledge with me anywhere in the world would be incredible.

And if “understanding the streets” includes knowing good restaurants or places to visit and whether a tourist would be welcome there, then all the better.

Language, because I can work the rest out and learn them more quickly than I can learn a language.

Absolutely the language any day. Knowing the language is the hardest step