Case in point:
I was advised by our guide that when a price was identified (on a trip south for a holiday), that price a vendor provided should be treated as 'the price' not an invitation to haggle. He explained that the local folk tended to set fair prices and that haggling was a sign of disrespect.
I'm not sure if it was simply a statement to get us to pay whatever price was indicated, but I also felt that it could have been accurate in depicting the folks we met selling stuff.
On the other hand, on another trip, I ran into the most uncomfortably in-your-face merchants I have ever met and they were aggressively competitive with one another for your dollar. It was exceptionally off-putting to someone who just wanted to walk down the street to the beach. It may have been moreso because I'm a Canadian and any haggling would be fairly gentle here, but there, they were up in your face and just did not take being put off with a gentle demure. That actually insured I would never return to that particular country again, although I feel certain that the pushiness was a result of economic conditions more than any cultural nature (other folk from there that did not engage in sales-to-live were much more easy going).
There are also differences in usage.
For instance, I found it quite surprising to find educated, nice folk in the Netherlands who were from Britain exchanging verbal good-natured jabs that included calling one another c*** and tw**. Apparently the British (or at least English) sense of those words was far less instantly offensive as it would have been in Canada or the USA. In Canada, if I'd said one of those to a friend, a stony look and a rebuke would be the least I would get and getting drifted or a real dressing down would certainly be conceivable.
ObTrav: When you said something you thought was harmless in Aslan, you might be surprised to find the Alsan you were speaking to reaching for a sword hilt. Or that Vargr merchant you were talking to using your 'Easy Vargr' guidebook and your best attempt to get the tonalities and harmonics right.... he might be rolling around on the ground howling in amusement at the incredibly off-colour thing you unknowingly said.
And then there's the issue of the 'trunk' in vehicles in UK vs. North America. And we've already talked about 'pop' and 'soda' and I could add 'hamburger' and 'wimpy burger' (NOT THE SAME!).
And then we throw in the Scots... or the Irish.... or Kiwis.... or Aussies. or the myriad of English and American accents (when I have gamed with people from the US Deep South online, I have found a whole different lexicon that would not be understood by my Scots cousin (and vice versa)). We are all English speakers in some sense, but our dialects, argots, cants, and accents leave us 'separated by a common language'.
There certainly could be some fun in building some local linguistic or cultural gotchas into different worlds.