In WordPress localization files (.po), does php map files by the line number eg. comments.php:60 or msgid msgid "<span class="meta-nav">←</span> Older Comments".
So basically, if I have said string in comments.php in line 60, and move it to line 74, does it still get localized by the msgid?
Yes it does. Localization does not depend on line numbers of strings.