The language of Call of Juarez: Gunslinger will automatically switch to the language setting applied on the console, provided the selected language is supported by the game itself.
Having said that, Call of Juarez: Gunslinger supports the following languages with respect to interface and subtitles:
English, German, French, Italian, Spanish (Spain), Russian, Japanese, Polish, Portuguese (Brazil).
Full audio voice-over is in English only.
To sum up, if the system language is set to, e.g. Polish and the game is supported in Polish, as indicated above, then the language of Call of Juarez: Gunslinger will be Polish.
In addition to the above, check Nintendo's information on how to change the system language, please.