How to make kite line more visible?
You will find a number of kite forums on the internet. Common to many is the consideration that one does not fly in areas known to have substantial air traffic.
This includes airports and hang gliding (and paragliding) sites.
Adding tails/ribbons to the line will not increase appreciably the visibility of the kite line to pilots, but it will increase the drag both for gaining altitude and for retrieving the line. Birds may be the only entity that would benefit from flagging the line. In the past years of kite flying, I've only had line contact one time, at the beach, by a seagull and it tumbled a bit and continued on its way.
You may want to consider to ask a moderator to migrate your three questions to the engineering SE as it appears to fit that category better than Arts &amp; Crafts.
Per the suggestion of another answer-poster-person, my comment to attach a battery operated/powered emergency strobe (typically 360° visibility) will improve the visibility of a kite. Aircraft have beacons for this reason and pilots will react more effectively to such a lighting device.
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