Localization and internationalization, what's the difference?

Internationalization (i18n)

the process of changing your software so that it isn't hardwired to one language/locale/culture.

Localization (l10n)

the process of adding the appropriate resources to your software so that a particular language/locale is supported. It's bigger in scope than just this Wikipedia entry, but it's a good start.

The value of distinguishing between them is that (theoretically) once your program goes through the i18n process, you can then iterate many l10n processes as you need them; also, it's nice to be precise with language.

According to Apple:

Internationalization is the process of designing and building an application to facilitate localization. Localization, in turn, is the cultural and linguistic adaptation of an internationalized application to two or more culturally-distinct markets.

Internationalization prepares your application for localization. For example, you might encode characters stored in your database in Unicode (utf8mb4 instead of latin1), moving strings to resource files, enabling the use of date, time and currency formats, etc.

When you wish to sell, for example, a Chinese version of your app, you'd then localize it by hiring a translator to build the zh-CN resource files, and use a new date/time/currency format.