"Une mer calme n'a jamais fait un marin chevronné."
Here's a breakdown of the translation:
* Une mer calme: A calm sea
* n'a jamais fait: never made
* un marin chevronné: a skilled sailor
Here are some other possible translations, depending on the nuance you want to convey:
* "C'est en mer houleuse que l'on devient un bon marin." (It's in rough seas that one becomes a good sailor.)
* "Une navigation tranquille ne forge pas un marin expérimenté." (Peaceful navigation does not forge an experienced sailor.)
Ultimately, the best translation will depend on the context and your specific intentions.