Problem I: Invalid langstrings

webserver_chrome:31375:[Wed May 08 07:34:58.521240 2019] [php7:notice] [pid 865] [client 172.21.0.13:54944] Debugging: Invalid get_string() identifier: 'strftimedatetime' or component 'core'. Perhaps you are missing $string['strftimedatetime'] = ''; in lang/en/core.php

2009 im:master> gg "'strftimedatetime')"

admin/tool/policy/classes/acceptances_table.php:                return userdate($row->timemodified, get_string('strftimedatetime'));

admin/tool/policy/classes/output/acceptances.php:                    $version->timeaccepted = userdate($acceptance->timemodified, get_string('strftimedatetime'));

mod/lesson/essay.php:                    $linktitle = userdate($essay->timeseen, get_string('strftimedatetime')).' '.

mod/quiz/report/overview/overview_table.php:        $this->strtimeformat = str_replace(',', ' ', get_string('strftimedatetime'));

mod/quiz/report/responses/last_responses_table.php:        $this->strtimeformat = str_replace(',', ' ', get_string('strftimedatetime'));