Bound by a common language and torrid history, 21-year-old Mexican nanny Lucila (Anna Diaz) bristles against her surroundings in Madrid, where she and her younger brother followed their mother from their Mexican hometown. “ Ashes ” shares these broad strokes with the novel on which it’s based — Brenda Navarro’s evocatively titled “Ceniza en la boca,” or “A Mouthful of Ash” — but even those unfamiliar with the book might still be tipped off to the haphazard nature of Diego Luna ’s adaptation.