The explanation by @pixelijn is correct. You will only need one license for all language versions and testing sites.
However you will need to register your license for each domain as Kirby will only consider the license valid if the domain matches. Make sure to use different config dirs, otherwise the license files will overwrite each other.
The license for one domain will automatically count for the staging and test subdomains though.
Alternative: Only use one of the domains for Panel access, then you only need to register the license for that domain. The Kirby core itself won‘t check the license.