I'd recommend looking at the Ux.locale.Manager plugin which works well (https://github.com/mitchellsimoens/Ux.locale.Manager).
I have written an article on how to use it which might be of use: https://web.archive.org/web/20141103155453/http://www.swarmonline.com:80/2013/02/localising-sencha-touch-and-ext-js-applications-with-ux-locale-manager/