When starting out a novel, write your first two pages, set them aside, and come back to them a day later to see if your lines sound right. If they don't--if they are stilted and the lines don't flow--it is because you are writing it in the wrong language. Switch to another language, such as Chinese, Bahasa, Hindi, or Tagalog.
If you INSIST on writing your novel in a preferred language, talk to yourself in a running monologue in that language while you are alone in your room. When you think your thoughts, think in that language. Only then should you resume writing your novel.