Quick search:

What is localization

localization is adaptation of software to users of a different culture
Localization is necessary because different cultures not only use different languages, but also different number formats, date formats, calenders etc. Php has a setLocale function that supports locales according to the  ISO 639 standard. The locale then is used by diffferent functions, or can be used to retrieve parameters for others. This means you have to know which functions to use and how to use them (if they are there at all - what is the inverse function of strftime?). StringConverter simplifies all this.

In order to localize an application several overrides must be made, including StringConverter settings. See 'Localization' in the 'How to' menu of the php website, and example 10.