According to W3C :

“Internationalization is the design and development of a product, application or document content that enables easy localization for target audiences that vary in culture, region, or language.

Localization refers to the adaptation of a product, application or document content to meet the language, cultural and other requirements of a specific target market (a locale).”

In other words,

I18n – Internationalization – Providing some kind of framework so that one can easily swap out strings, graphics, sounds and other specific resources and generally handling different conventions of communication.

L10n – Localization – The process of creating a set of strings, graphics, sounds etc. so that you can target a specific nationality, language or region. It also involves customizing elements like time and date formats, currency, accounting standards, culturally appropriate images, symbols, and hand gestures, as well as spelling and other locale-specific components. 

Internationalization is usually done first, before localization, as it lays the foundation for adapting a product to different languages and cultures.