Does your project require you to use internationalization? If so, it should be in your design. If you are assuming the app will only be used in one country, an assumption is good.
In my assumptions section, I wrote that I assumed that the design would be implemented by developers who knew how to do X, Y and Z. This allowed me to make brief mentions of things like "use JPA" without going into a ton of detail. I didn't need internationalization in my submission though.