Where does “plāns” come from?

plāns (Latvian) comes from German plan, from French plan, from French plain, from Old French plain, from Latin plēnus, from Proto-Italic plēnos, from Proto-Indo-European pl̥h₁nós, from Proto-Indo-European pleh₁- — to fill.

plāns (Latvian): plan, map, blueprint, layout; plan, map; plan