euzk[cad_] :=
 StringReplace[
  cad, {"á" -> "a", "é" -> "e", "í" -> "i", "ó" -> "o", "ú" -> "u",
   "ca" -> "ka", "ce" -> "ze", "ci" -> "zi", "co" -> "ko",
   "cu" -> "ku", "ch" -> "c", "cl" -> "kl", "cr" -> "kr",
   "cc" -> "kz", "cg" -> "kg", "cn" -> "kn", "cp" -> "kp",
   "ct" -> "kt", "ge" -> "je", "gi" -> "ji", "gue" -> "ge",
   "gui" -> "gi", "h" -> "", "qu" -> "k", "v" -> "b", "w" -> "u",
   "y" -> "i"}]

eusk = {"á" -> "a", "é" -> "e", "í" -> "i", "ó" -> "o", "ú" -> "u",
   "ca" -> "ka", "ce" -> "ze", "ci" -> "zi", "co" -> "ko",
   "cu" -> "ku", "ch" -> "c", "cl" -> "kl", "cr" -> "kr",
   "cc" -> "kz", "cg" -> "kg", "cn" -> "kn", "cp" -> "kp",
   "ct" -> "kt", "ge" -> "je", "gi" -> "ji", "gue" -> "ge",
   "gui" -> "gi", "h" -> "", "qu" -> "k", "v" -> "b", "w" -> "u",
   "y" -> "i"};

deusk = Dispatch[eusk]

euzk[cad_] := StringReplace[cad, deusk]

StringReplace::srep: Dispatch[Length: 27] is not a valid string replacement rule.