no return

_.mixin({
    renameProperties: function (object, translations) {
        _.each(_.pairs(translations), function(translation) {
            _.each(object, function(item){
                if (item.hasOwnProperty(translation[0])) {
                    item[translation[1]] = item[translation[0]];
                    delete item[translation[0]];
                }                             
            });
        });
    }
});

usage

var collection = [
    {
        "prop1" : "val",
        "prop2" : "val"
    },
    {
        "prop1" : "val",
        "prop2" : "val",
        "prop3" : "val"
    },
    {
        "prop2" : "val"
    }
];

var translation = {
    "prop1" : "property1",
    "prop2" : "property_two"
};

_.renameProperties(collection, translation);

with return

_.mixin({
    renameProperties: function (object, translations) {
        _.each(_.pairs(translations), function(translation) {
            _.each(object, function(item){
                if (item.hasOwnProperty(translation[0])) {
                    item[translation[1]] = item[translation[0]];
                    delete item[translation[0]];
                }
                return item;                                
            });
        });
        return object;
    }
});