[[MediaWiki:Flaggedrevs-protect-legend]]

This message should probably use int: instead of MediaWiki:. The automatic checker does not consider the link properly translated if the language code is appended to the pagename. Check MediaWiki:Flaggedrevs-protect-legend/pt for an example.

No, it should not use int: instead of MediaWiki:. Within the same instance of MediaWiki the documentation entry point should be static between user interface languages, and not dynamic.

Should the /pt message have {{MediaWiki:Validationpage}} or {{MediaWiki:Validationpage/pt}}?

It should not include the /pt. For links and transclusions in the MediaWiki: namespace, it will automatically use the language set in the user's preferences. Test it in some wiki's sandbox some time. :)

Thanks