One of the sites I regularly visit is Query Shark - a blog site for 'query letters' touting stories. It's run by American Janet Reid and, if you're not familiar with it, she comments of query letters sent to her as to their style, content and overall quality.
It's great and a must-read for all novelists - wannabes and established.
She doesn't pull any punches; her advice is straight-to-the-point and quite often brutal, but that's as it should be. In the real world prospective agents or publishers wouldn't be any different and it's a way to learn how to hone the query letter to make it short and interesting.
One query that I read recently (#123) was from a British writer and the language in his/her query provoked many comments from the mainly American readers - mostly along the lines of that they couldn't understand what the author was meaning or whether they were genuine words. One didn't understand 'John's out the nick', 'handing out money to all and f*cking sundry' and 'blowing up the bastard screw'. Many couldn't understand what they called 'the slang' and ended up confused.
I think that in the UK we've had so much US tv shows and Hollywood films over the years that we've become accustomed to 'America-speak', and understand pretty much all of what a US author writes, but transpose that and they seem to hit a wall. And they also seem to think that slang is centred only on London and within the sound of Bow bells at that ...
So what can we learn from this? If we want to appeal to a wider market (i.e. the US) do we have to curtail the use of colloquialisms and 'dumb down' the language? Or - like Guy Ritchie - beat them round the head with the stick of rhyming slang? Personally I think a mix of the two is the ideal - use a colloquialism but show the meaning through a visual description. I don't think that American readers will baulk at UK-slang if they're shown what the word, or description, means. I'm sure that some deep-south US slang would leave us all baffled on this side of the pond, but if it were 'explained' through action then it wouldn't seem out of place.