These two terms are sometimes hard to tell apart. Here is a short clear definition from 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.
So, when you re-factor your code and start putting the UI text into a language resource file, that process is called internationalization. After that re-structuring process, when you go look for a translator to populate another language resource file, that process is called localization.