Not that it addresses the ability to comment out an action, which I too think would be a nice feature, but for troubleshooting a Translation Error, I find it easier to use a debug statement versus removing line by line. That is, put a Copy @null to @null action with a tcl ‘puts’ statement just to echo out a debug message. Then using the Xlate testing tool, if the echo message does echo out, the problem is lower in the Xlate, so I move the debug statement lower (kind of using binary sort method – put the debug in the middle of the Xlate, then in the middle of the section you check next, depending on if the debug tells you the issue is down further, or back up higher).
