I'm writing a letter in French to my fiancé who is originally from a French speaking country. We usually communicate in English but I want to impress him by writing "Don't die before I do" in French. Thank you very much in advance.
Without more context, I suggest "Ne meurs pas avant moi."(=> literally don't die before me) You can't have a word for word translation because translating I do in French would be redundant and sound clumsy at best.
Now, there are many other ways to express the idea of death. Like :
Quitter ce monde/cette Terre
Rejoindre les cieux/l'au-delà
And much more.
But since I don't know the general tone of your letter, I can't say if they would be appropriate to use.
Taking your last comment into account, you can use the expression survivre à quelqu'un to express the idea of outliving someone.
I want you to outlive me can thus translate to Je veux que tu me survives. But Je veux can sound a bit harsh so I would tone it down a bit and say Je souhaite que tu me survives (Je souhaite => I wish).
Something you see quite frequently in dramas when someone is dying and their significant other don't want them to die is:
Ne me laisse pas seul(e) !
(lit. : "don't leave me alone"). However in your context (you writing to your girlfriend) it doesn't work unless she is really dying. For your very context there is no French expression that I know that is as common as "Don't die before I do". You will have to make something up. Anyway all the ways I see to express this idea sound terribly corny, way more than the English expression:
Ne meurs pas avant moi ! (direct translation)
Attends moi pour mourir ! (sounds a bit better imho)