That's usually when you can think in that language. Forming sentences doesn't feel like pulling teeth from an angry ogre anymore.
You start to enjoy yourself.
When you have the basics down, you get hungry for more: all those delicious idioms and sayings, those quaint turns of phrase. It's not enough that you can communicate adequately; you want to show off. I know what this means, just like the natives! Hey, just watch me use it in a sentence!
This is as good as it gets. You give yourself a mental high five every time you make a joke or use that neat new phrase.
Idiomatic language is the dream; it's what we all want to learn.
Because that's how people talk, right?
Then you find out that none of this applies to writing.
No, with writing, you need to be original. You can't use those shiny little metaphors you've picked up.; you have to make up your own. You know, something new and different. Not cliché.
Now you're approaching that ogre again, clicking your pliers. Only now the ogre sees you coming and runs away, throwing boulders the size of houses at you. In a bog. In the middle of a snowstorm. And one of your legs is broken.
You see where I'm going with this, right?
But there is a way to use those phrases and clichés; just give them a little twist, and they seem fresh again.
Joss Whedon is a master at this. Here are some of my favourite quotes from Firefly that illustrate the point:
"Well, my time of not taking you seriously is coming to a middle."
Mal: "Ah, the pitter-patter of tiny feet in huge combat boots..."
Mal: "Looks like business ain't runnin' so much as crawlin' away."
Go on, go watch Firefly or Buffy again. You know you want to. It's for educational purposes this time. (As if you need an excuse!)
Here's a challenge for you: whenever you come upon a cliché, try to think of a way to make it more interesting.
So do the twist.
It's fun and good for you.