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. The film plays out like a tale where too much has been relegated to the margins and left between the cuts, where the performances shine but their emotional foundations have been laid in reverse.   First-time filmmakers can usually be forgiven some of these sins, only most viewers might not realize that this is Luna’s fifth go in the director’s chair.…