First off, I apologize for how late this post is. I kind of forgot it was Monday today and not another Sunday because of the long weekend in the States/Canada.
Exploration is a key part of RPGs. Over the course of a campaign, whether it be Fantasy, Modern Day, or Sci-Fi it is likely only a matter of time before your players come into contact with a culture that is different from their own. Some games thrive on this. L5R, for example, is all about giving a "western" perspective on East Asian culture and mythology from their own middle ages. However, regardless of the cultures you are presenting, there are some things that are good to keep in mind.
From The Outside We're All Strange
The first thing to keep in mind is that cultures different from our own always seem strange to us. This is ok. It is perfectly natural. However, just because something is strange does not mean that it is better, worse, or done without reason. It is just different. There will be places where things are similar, places where things are different, and places where at first glance it looks to be almost an exact match or completely the opposite only for parts of it to go the other way.
The important thing here is to look for the reason behind the differences. If you know why something is the way it is, it seems less strange and that will help with portraying the custom or ritual as a more normal and matter of fact thing.
Strange Does Not Mean Funny
We have a tendency to ridicule things that are different from us. Things seem strange, and thus they're different and wrong, and from there we get jokes. If you are bringing in a different culture into your game and want it to be treated seriously you need to avoid doing this. Don't comment on how strange things are. Don't make jokes. Don't say the people are backwards or funny in how they do things. Treat the culture with respect. Remember, to that culture everything they're doing is completely normal and what the players are doing, how they act, is the "weird" thing being perceived. This is especially true when it is the players visiting the lands of this foreign culture and not the other way around.
Culture Clash Can And Will Happen
Clashes of viewpoint are always going to happen when you have two cultures mixing. I can't tell you the number of times I've had to explain small cultural things to new players to L5R only for them to rail against it because it didn't make sense to them and their view of the world. The acceptance of paradox. The fact that testimony trumped evidence in investigations. That you couldn't be direct in asserting things because it seemed too strong, too bold, or too rude. These are common occurences in L5R play, and similar things will also happen should you have multiple cultures in your game.
A Guide Is A Good Thing To Have
Lastly, if you are dropping your PCs into this strange/foreign culture, than it is a good idea to give them a guide. Someone they can ask questions of. Someone that can help cover or explain their social faux pas. Introduce them with the guide to the culture, and then let things go from there.