Internationalization is making multiple versions of a s/w to suit natural and/or cultural and/or linguistic requirements of different regions around the world.

Localization is making the required changes into a s/w of a particular region (say England) to suit natural and/or cultural and/or linguistic requirements of the local region (say India).

In the testing phase, there would be respective testing for these 2 concepts to see if they are carried out correctly and completely. As it is evident from the above definitions, such a test team needs to have good knowledge of the transformational requirements (nature, cultures, languages) involved.