lst = DictionaryLookup[{"Spanish",
x__ /; And @@ (StringCount[x, Alternatives["a"#] == 1 & /@ {"a", "e", "i", "o", "u"]] == 5"u"}] )}];
Short @ lst
{ababillarseabrenuncio, abadiadoaceituno, abadiatoacudidero, abajaderoadoquier, <15405>><< 447 >>, zurrapientavomipurgante, zurrapientovulnerario, zurriagazoyeguarizo, zurrapiento}
Length @ lst
15412455