string[s_, n_] :=
  Module[{lista = {}, i = 1, m, j, str, strcar = Characters[s]},
   While[i <= Length[strcar] && i + n - 1 <= Length[strcar], j = 1;
    str = "";
    m = i;
    While[j <= n, str = StringJoin[str, ToString[strcar[[m]]]];
     ++j;
     ++m;];
    AppendTo[lista, str];
    ++i;];
   AppendTo[lista, Reci[n]];
   	Return[Flatten[Sort[lista]]];
   ];

Reci[b_] :=
  Module[{i, l = {}, ld = IntegerDigits[b]},
   For[i = 1, i <= Length[ld], ++i,
    Switch[Mod[ld[[i]], 10], 0, AppendTo[l, "nula"], 1,
      AppendTo[l, "jedan"], 2, AppendTo[l, "dva"], 3,
      AppendTo[l, "tri"], 4, AppendTo[l, "cetiri"], 5,
      AppendTo[l, "pet"], 6, AppendTo[l, "sest"], 7,
      AppendTo[l, "sedam"], 8, AppendTo[l, "osam"], 9,
      AppendTo[l, "devet"]];];
   Return[l];];