The Regular License does not allow for using a theme multiple times on a WP mult-site install. I’ve done the same as duotive, letting it go for language variations. In my opinion using the tools available to create multiple language versions of the same site is one usage. If the changes went beyond just language and it was a truly different site for different regions, like what you might see on Panasonic’s websites:
- http://www.panasonic.de
- http://www.panasonic.co.uk
- http://www.panasonic.fr
Although the design is the same, it isn’t just the language that changes, it’s the content also. They’re running a different targeted website campaign for each language/region. In a case like that I would tell the custom they need a license for each site.
It is explained pretty will in the License Usage Examples:
http://themeforest.net/licenses/usage_examples#wordpress_regular