Keeping languages separated is always going to be challenging, especially when you're talking about closely related languages like the Romance set.
I do the following to try and keep them as separated as I can:
- Pay SERIOUS attention to pronunciation and ear training. Having very different sound-sets in my head is my first defense against language mixing.
- Never re-use images on my anki cards across languages.
- Never re-use mnemonics on my anki cards across languages.
- Never study related languages at the same time.
- Give each language at least a year to really solidify.
Beyond that, it's just a matter of getting lots of exposure, lots of sentence study, and hard work. You ultimately save time when learning related languages; you'll progress faster...but you definitely will pick up mistakes that specifically caused by mixing, and the only thing you can do to fight them is more exposure.