Instead of modifying the value of the string, it would be better to use a Global string:
JGRID_HEADING_ID="ID"

Tested the patch - it's working!
Column ID for Updatesites is now "ID".

Also checked that I can override the template string "JGRID_HEADING_ID" - is not working.
So I guess there is not yet a global string.

Is that going to change?
If not, I can set me test result to successful.

Please let me know.

@seagul30
The suggestion is to change the code to use JGRID_HEADING_ID instead of modifying the value.
I.e. change in ROOT/administrator/components/com_installer/views/updatesites/tmpl/default.php, line 69:

<?php echo JHtml::_('grid.sort', 'COM_INSTALLER_HEADING_UPDATESITEID', 'update_site_id', $listDirn, $listOrder); ?>
to
<?php echo JHtml::_('grid.sort', 'JGRID_HEADING_ID', 'update_site_id', $listDirn, $listOrder); ?>

it gives the correct rendering:

This is true even in the screen "Extension Manager: Update Sites". The column heading for the ID is "Update Site ID".

Shall we also make this as ID? If so, a separate Issue Tracker has to be created?

There was a discussion at the time about this. Sorry I am not at my PC so
can't find it but it is on the tracker somewhere
On 4 Mar 2015 05:34, "imanickam" notifications@github.com wrote:

This is true even in the screen "Extension Manager: Update Sites". The
column heading for the ID is "Update Site ID".

Shall we also make this as ID? If so, a separate Issue Tracker has to be

created?

This comment was created with the J!Tracker Application
https://github.com/joomla/jissues at issues.joomla.org/joomla-cms/6075
http://issues.joomla.org/tracker/joomla-cms/6075.

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#6075 (comment).

I would agree with JM.
The JGRID_HEADING_ID should be used. And the COM_INSTALLER_HEADING_UPDATESITEID string can be removed from the language files.

PS: Same thing for COM_INSTALLER_HEADING_ID.