Have you ever mixed languages up? How do you prevent this?

If in learning Spanish you didn't have problems with mixing English and Spanish, you should have no problems with Italian because you'll be using a different frame of reference. I always thought I wouldn't be able to learn French after I had learned Spanish. I'm not fluent in French and what I know has never been mixed with Spanish at which I am fluent. Every once in a while, I will be speaking English and the Spanish word will come to mind and drive away the English word causing me to falter momentarily and laugh at the gaff. Learn on.