For translate constants in Simple membership show "users form" ordasoft.com/os-simple-membership-joomla-membership-extension
You need go to Joomla Language manager,
admin->extensions->Language->override, select "site" and press "new"
next you need do search by value, every constant which you need replace:
- registered users
- Online status .......
And create, new override for every.
For change data format
find and replace row
echo "<td> " .JHtml::_('date', $user->lastvisitDate, 'Y-m-d H:i:s') . " </td>
Please backup file first