When employees at the Marlborough Senior Center looked around a few years ago, they realized that the people who were showing up for the “blue plate special’’ lunches on Mondays and the weekly cribbage games did not look like the community outside its doors. “We knew our community is comprised of a lot of Portuguese families,” said Jennifer Claro, the center’s executive director. “We knew there were a lot of seniors that we weren’t seeing in our center.”