Manual:$wgNoFollowDomainExceptions

Details
If this is set to an array of domains, external links to these domain names (or any subdomains) will not be set to  regardless of the value of nfl>Special:MyLanguage/Manual:$wgNoFollowLinks|$wgNoFollowLinks.

For instance:

This would add  to links to de.wikipedia.org, but not en.wikipedia.org, wiktionary.org, en.wiktionary.org, us.en.wikipedia.org, etc.