Proposed hide some untranslatable code through covering by <tvar name="$1"> (with the creating/changing documentation):
link to Special:MassMessage
wikilink to :mw:Special:MyLanguage/Extension:MassMessage
external link to https://www.mediawiki.org/wiki/Special:MyLanguage/Help:Extension:MassMessage
substitution {{int:htmlform-submit}}
substitution {{int:massmessage-form-submit}}
replacing [[Special:Permalink/$2|$1]] to the $1