Although localization and, by extension, localization testing are not strictly a part of the development of world-ready software, localization becomes possible once you have developed world-ready software. If you do decide to localize, you should be familiar with the scope and purpose of localization testing. Localizers translate the product UI and sometimes change some initial settings to adapt the product to a particular local market.
Adequate testing ensures that you deliver a high-quality product, catching—and correcting—any bugs that come up in the localization process before it’s released. It saves you time and money, too. After all, finding these errors post-release can mean costly rework and having to address them multiple times (for each target language).