I usually create a class with a "msg" object that contains all the messages and text that I want to show in my components.
mymodule.js
Code:
var App.mymodule.Main = {
msg : {
title : "Welcome to this module",
name : "Name",
lastname :"Last Name",
.......
},
init : function(){
var win = new Ext.Window({
title : this.msg.title,
.......
})
win.show();
}
}
When I want to select another language I just override the msg object including a file with the translations, like this:
mymodule-es.js
Code:
Ext.override(App.mymodule.Main,{
msg : {
title : "Bienvenido a este modulo",
name: "Nombre",
lastname: "Apellido",
.....
}
});
This file only contain the translations 
I hope you can understand the main idea, my English is no so good