Ideally, the translation needs to happen before the UI design. I've seen a lot of UI designs come straight from the designer with a beautiful pixel-perfect depiction of controls, but assuming English. So the "MENU" button was designed deliberately such that exactly four latin characters fit inside of it horizontally. Or assuming people's names fit in one line of text, or addresses have a certain number of lines and so on. Then when you get around to translating everything, the design has to go back to the drawing board.