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.
This definitely reduces the "world-readiness" of the application. That is, a globalized application whose UI and documentation are translated into a language spoken in one country will retain its functionality. However, the application will become less usable in the countries where that language is not spoken.
Localization testing checks how well the build has been translated into a particular target language. This test is based on the results of globalized testing where the functional support for that particular locale has already been verified. If the product is not globalized enough to support a given language, you probably will not try to localize it into that language in the first place!
You should be aware that pseudo-localization, which was discussed earlier, does not completely eliminate the need for functionality testing of a localized application. When you test for localizability before you localize, the chances of having serious functional problems due to localization are slim. However, you still have to check that the application you're shipping to a particular market really works. Now you can do it in less time and with fewer resources.