Have you guys heard of English to English? It's an LJ com that isn't very active any more, but it's for fanfic writers to help one another be more accurate in their dialogue.

Here's my take. (I'm US.)

When writing the prose parts of the story, spell in your native tongue. To try to change every "color" to "colour" is just tedious and you're not likely to catch every use that needs changed. So, write the prose as you normally would, unless the phrase is glaring, like the one mentioned above. Those can be changed if needed to help the story seem more like it is set where it's supposed to be.

But the dialogue? That matters. Clint would not call "chips" "crisps" and the like.

I write in several fandoms from the UK (most notably Being Human and Sherlock and less so Harry Potter) in which the characters must sound like themselves. I try very hard to make that happen. Thus, when I am unsure if a forty-year-old man would use the term "stroppy," I ask a British friend. I'm pretty darn fluent in Brit, as I write a bit for a UK magazine (fortunately, my editor lets me write in Yank and changes it for me), but just a year or so ago, I had to correct "clerk" for "cashier," and "store" for "shop" still slips through sometimes. Fortunately, my readers know to PM me and help me fix such things.

Because it does matter. It's not a big deal sometimes, and for a very good fic, it's just a distraction, but sometimes it just pulls me out of a story too much when a character doesn't speak like they should.

It's like Joss said in the commentary: "I'm an atheist, but Steve Rogers isn't." I'm American, but Harry Potter isn't. Just as I don't speak like Malcolm Reynolds (much), he doesn't talk like a Yank.

But I digress and I'm rambling. I should get to bed.

Fun post, this. I enjoy these types of discussions.