Friendly picky correction for several different ways of writing it:

  French: Plaisance
  Occitan: Plasença
  Spanish: Plasencia

And some extra trivia: There are a bunch of different French towns named Plaisance, some of them near the Pyrenees, so probably that where the Plasentia name came from, as I saw that it was French and Basque sailors the ones that arrived there.

The Spanish city of Plasencia is quite far away from the Basque Country, in Extremadura, so probably they weren't referring (or remembering) that concrete city.