Hide product fields

This is actually a bit tricky. I will look for ways to improve it, but the following should do the trick for you.

1- You will need to configure your product fields to "Allow field to be populated dynamically" on the advanced tab. Enter unique parameter names for each field. Following are the parameters I used for the code snippet below: http://tardis1.tinygrab.com/awEm

2- Use the following code snippet, replacing the IDs to match your form and fields.

add_filter("gform_field_value_prod1_name", "prod1_name");
function prod1_name($name){
   return "new product name";
}

add_filter("gform_field_value_prod1_quantity", "prod1_quantity");
function prod1_quantity($name){
   return 3;
}

add_filter("gform_pre_render", "populate_product");
function populate_product($form){

    //only setting product for form id 165
    if($form["id"] != 165)
       return $form;

    //Setting product field id 8. Replace 8 with your actual field id. You can get the field id by looking at the input name in the markup.
    foreach($form["fields"] as &$field)
        if($field["id"] == 8){
            $field["label"] = "new product name";
            $field["basePrice"] = 22;
        }

    return $form;
}

I understand pre-populating product fields are not very developer friendly right now. This will be improved in our next major release.
For the validation hook, you need to access the price and quantity from the $_POST variable.
Following is an example to point you in the right direction.

//Replace 165 with your actual Form ID
add_filter('gform_validation_165', 'custom_validation');
function custom_validation($validation_result){

    //finding field with ID of 8 and making sure quantity is greater than 0
    foreach($validation_result["form"]["fields"] as &$field){

        //NOTE: replace 8 with the field you would like to validate
        if($field["id"] == "8"){

            //Base prices are retrieved using "input_FIELDID_2"
            $base_price = GFCommon::to_number($_POST["input_8_2"]);

            //Quantities are retrieved using "input_FIELDID_3"
            $quantity = $_POST["input_8_3"];

            //fails validation if quantity is not a positive integer or if price is less than $20
            if($quantity <= 0 || $base_price < 20){
                $validation_result["is_valid"] = false;
                $field["failed_validation"] = true;
                $field["validation_message"] = "This field is invalid!";
            }

            break;
        }
    }
    return $validation_result;
}

Regarding the translation of the Price and Quantity sublabels, you can translate them using the translation files with PoEdit. Following is a good article explaning how to do that. http://weblogtoolscollection.com/archives/2007/08/27/localizing-a-wordpress-plugin-using-poedit/
If you don't find those strings in your translation file, you may need to update it from the gravityforms.pot file. You can do that in poEdit by clicking on Catalog -> Update from POT file. http://tardis1.tinygrab.com/awKN

The next major release will be 1.5.3. We are still collecting requirements for it, so I imagine it will be around 3 months, maybe a little more.