In superset/translations/messages.pot
we find :
#, python-format msgid "Added to 1 dashboard" msgid_plural "Added to %s dashboards" msgstr[0] "" msgstr[1] ""

%s should be present in msgid and msgid_plural like :
#, python-format msgid "Added to %s dashboard" msgid_plural "Added to %s dashboards" msgstr[0] "" msgstr[1] ""