Depends on what you're writing. If it's an informal message to a friend, sure. If it's something for work or school, or anything that you intend to give to a superior of some kind, I would suggest you use the proper form of the words. If you where writing a story though, you could use informal words in the characters speech. That's quite common in books, especially if the author wants the characters to feel natural. I hope this helps.

Unless you are a novelist trying to depict an uneducated character in your book, NO.

No. Not unless you are referring to writing text messages to your friends, and even then, it is sloppy English, and it sounds juvenile and uneducated. If you are going to all the trouble of learning another language, please learn to speak (and write) it well. Good luck!