I18N - Internationalization
Drupal is, thankfully, fully setup for easy internationalization. Careful adherence to the t() function in the display of all strings helps to ensure that.
Localization must be dealt with per locale as per your own marketing strategies. My own experience at international trade events is that most people prefer English, if they cannot get their own language. Thus, it follows that locales should follow a target population base, rather than trying to find a compromise.