There are some ongoing problems with update of language strings. In 1.7 are some strings in lang/en_utf8 deleted. They are part of the lang/en_utf8 package in 1.6.x. We are creating new language strings in 1.7 systems and old strings are automatically deleted.
After an update of the language files in 1.6.x in all other languages there are several strings missing, because they are deleted during actual translations.

It seems that some of the strings are not used in 1.6.x systems. So they should be deleted in the 1.6.x systems in lang/en_utf8 folder. Others can'tbe deleted.

Here are the problem strings:

moodle.php
$string['statsreport6'] = "All teacher activity (views and posts)";
$string['statsreportstudentactivity'] = "All student activity (views and posts)";
$string['statsreportteacheractivity'] = "All teacher activity (views and posts)";

assignment.php
$string['allowmultiple'] = "Allow multiple files";
$string['attachfile'] = "Attach file";
$string['attachfiletoassignment'] = "Attach file(s) to assignment";
$string['backtoassignment'] = "Back to Assignment";
$string['backtofeedback'] = "Back to Feedback";
$string['choosereviewfile'] = "Choose a file to upload as an assignment review:";
$string['deletecheckfile'] = "Are you absolutely sure you want to delete this file?";
$string['deleteednotification'] = "File has been deleted.";
$string['deletefail'] = "The following file has not been deleted:";
$string['markingsubmitnotification'] = "File has been submitted for marking successfully.";
$string['namedeletefile'] = "You about to delete this file:";
$string['nofilesforsubmit'] = "No file was found for submitting - are you sure you uploaded any?";
$string['removelink'] = "Remove";
$string['responsefile'] = "Response File:";
$string['responsesfromteacher'] = "Responses Uploaded by the Teacher";
$string['step1'] = "Step 1: ";
$string['step2'] = "Step 2: ";
$string['submissionstatus'] = "Submission status:";
$string['submissionstatusblank'] = "Blank";
$string['submissionstatusdraft'] = "Draft";
$string['submissionstatusmarked'] = "Marked";
$string['submissionstatusreturned'] = "Returned";
$string['submissionstatussubmitted'] = "Submitted";
$string['uploadsuccessresponse'] = "Response file uploaded successfully.";

blog.php 1 String

hotpot.php 2 Strings

lesson.php 7 Strings

And there is an ongoing problem with currencies.php. There are some old currencies in the en_utf8 file that should be deleted. Nobody can pay in old 'historic' currencies.