I suspect that it's because http://www.trackandroad.net/pages/creerRoadbooks/V3/creerRoadbookCalculRoute.js file checks text in French or English like in

recherche=/(.*)Continuer(.*)sur(.*)/;
rechercheEng=/(.*)Continue (.*)on(.*)/;
if (recherche.test(instructions) || rechercheEng.test(instructions))
{
image=2;
}

and my instructions appear in Spanish ...

There is an undocumented property of Google Maps DirectionsStep object called "maneuver" which can be used to select icon.
Values are language independent.