Product Cycle: Setting a milestone to freeze all the UI elements of your product

In a typical Product Development model, the focus is on getting the newer features into the product. So, in the normal model, you have a list of new features that is decided in collaboration with Product Management and these are implemented over a period of time. The timeframe for this implementation can vary depending on the type of product development model being followed such as Scrum, Iterative, Waterfall, etc.
However, whatever be the cycle you may be following, it is critically important to have a specific milestone in your entire schedule where you no longer make a change to the UI components of your product. This could mean the layout of dialogs, the artwork such as icons and others used in your application, the error messages and text on dialogs, and any other such UI elements.
And why do you need to have such a milestone in your schedule ? Aren't you limiting the amount of changes you can do if you need to have such a time when you can longer make changes ? Well, there are certain dependencies that most products have. For product being sold internationally, there is a need to ensure that your product is translated into the languages in which it is being sold, and to do this translation and testing of the different language product takes time; now if you keep on changing some elements of your UI, this process of getting your product converted into an international version will become very inefficient and costs will increase, so the simpler solution is to ensure that you freeze your UI elements at some time, which gives enough time for the translation, and also allows the product QE to get a final set of specifications and design that they can test against.