I think there is no harm done throwing in simple local terms and a few hard-to-figure-out ones into the narration to mark the whole story as the author's. Like, if I say "Hell sure...", my editor always edits it to "hell as sure". Of course I don't blame her since I didn't know it's a local way of saying it. I reject the edit, seeing that it wouldn't cause any confusion anyway. If the terms may be hard to figure out, try to throw it in a context with some details. For example...
People mostly say "sub", but I always call it "hoagie" because "sub" is never part of my daily vocabulary. I would just simply introduce that word when the character goes, let's say, to Subway and orders some "turkey hoagie with swiss cheese and on toasted bread". People should figure out that... Subway pretty much serves "subs", but the author/character calls it "hoagies" nevertheless.
Graeme threw in a few Aussie terms that kept me wondering. Especially when he repeated "ute" so many times... It took me a long time to find that word.
As for the author's thoughts. I always tend to keep it in the present tense when the rest of the narration is in the past. Few people pointed this out, but I kept it like that seeing that it would be my mark of style.
Mainly, narration should be simple, conscient, and clear, but do not complicate it throwing tons of terms and odd structure that most readers wouldn't say daily. I would just stop reading the story because I would lose interest getting out an urban dictionary on google every five minutes.