How to avoid missing anything when preparing your game for localization

Localizing a game is not just about handing over a text file to a contractor or community. There are many steps that need to be taken to make the game look natural and understandable in any language. After all, that is the main goal of localization.
When should you prepare your game for localization? Who should you hand it over to? What should you pay attention to? Time formats, line lengths, separators, and variables - do they all work the same for all languages? That's what we'll talk about.