Just the regular JoomSEF license. It does not matter you will you separate URLs as long as it is single Joomla site.
For registration, use the primary URL you will use to access administration. So e.g. if you have mysite-en.com for English and mysite-it.com for Italian, and you access adminsitration through mysite-en.com/administrator, for registration, use mysite-en.com.
In case these would be 2 separate Joomla installations - one for English and other for Italian, each with separate administration, you would need to get 2 JoomSEF license for each of the Joomla sites.