I am by no means bilingual (whatever that means...I'm still trying to figure out what "fluent" means) but I write articles in foreign papers and cultural reviews. (Quite a bit of it about American pop culture, which is considered pretty hip over a lot of the seas) but also political commentary. It's a pain in the ass. Especially accent marks. I just hate them. Thank GOD for spellcheckers, which can go through and get most of them.
There are several different ways of going about different tasks here. One is just to write the same way I write in English, then translate it, including the best shot I can get on the slang, etc. This works best doing something about popular American music. ( Es interesante que los apodos "Rock and Roll", "Boogie" y "Jazz" fueron todos sinonimos por tener relaciones sexuales. )
Another mode is to attempt to use the diction, etc. of the readers of the piece. Intellectuals is easy (once you start talking college stuff, all the Romance languages are just slipping over into Latin anyway) but hipsters is another matter. ( No es mas chido decir, No manches, wey? que "Me Vale"? ) Not for beginners. I get a boost from editors on this sort of stuff.
A third level would be writing as though you were actually a resident or native of the place you are writing in, for and about. A very tough one. One of my slickest tricks was a series of articles I sold to an American paper in which I posed as a foreigner and wrote in English, but in such a way that people familiar with the country I was pretending to be a native of would beleive that it had been written by a native. Very interesting exercise.
My favorite story of this, by the way is Jerzy Kosinsky, who wrote only in English. When he arrived in New York and started work on Steps and The Painted Word, both of which would win major literary awards, including the National Book Award for fiction, he would call up telephone operators and read passages to them, asking them if they sounded right.