I have inherited and changed the warning message in sale.py file. It's working properly. Sale order and Quotation also a same form view and mostly same functionality.

The function you're looking for is defined inside the sale_order_line class, more precisely in addons/sale/sale.py, line 860.

Thanks for your help, but i've already tried to modify the sale/sale.py (located in /usr/share/pyshared/openerp/addons ), to ensure that's the product_id_change function was the good function, i've modified the osv error raised when no customer id is selected, restarted the server but when i select a line without selected a customer the osv error is still the original and not the error i've modified. I've also tried to modify the error into the i18n/uk.po but the result is the same. I'm not sure that i'm doing the best thing.

I've another question linked to my problem : i want to know if the product_id_changed function located in the sale/sale.py file is used only for the sales orders form or also for the quotations view? Because i've seen that the same sale.order.form view is used, thinking it's inherited for both.