Je pisse dans le lait. (Literal translation) However, "Me cago en la leche" is a Spanish expression used to show annoyance or frustration, and is not usually translated literally. A more natural equivalent in French would be: Nom de Dieu ! or Merde ! or Putain ! Depending on the level of vulgarity desired. If you want to keep closer to the literal meaning (but it's not a common French expression): Je chie dans le lait. But again, this would sound odd to a French speaker. The best is to use a common French expletive as suggested above.