the old westerns were not recorded with sound usually then the audio done later in a studio by the same actors, so maybe thats where the confusion arises for you? some italians were dubbed over in english, but principal actors are english speakers already
In Italian westerns, there were a mutliple of different nationalities on the set who tended to speak the lines in their own langauge and then dubbing was made as appropriate for each market. English is the best (and most appropriate) language to watch this in because most of the dialogue is spoken in English by American actors who then dubbed their own dialogue. Thus, the English print features Fonda, Bronson, Robards, Lionel Stander, Frank Wolff and Keenan Wynn speaking in their own language. In the Italian print you will get Cardinale and Gabriele Ferzetti (who are dubbed by others in the English print) speaking their own language as well as the Italian bit players.