I travel to France quite extensively, and I was worried about the lights too.
I emailed SEAT, who replied (as expected) saying that the lights were suitable for use abroad with no adjustments. I then just ran their email through Google Translate into French, printed it and shoved it in the glovebox.
I also carry a totally useless set of spare bulbs (from my old Vectra), just in case Monsieur Plod wants to see those. I figured if a copper is too daft to realise that the Leon doesn't need spare bulbs, he'll be too daft to realise the ones I proffer won't fit...