I'd like to translate a line in checkout > payment information but I dont' find the CSV file. I tried to add the translation on mage_sales.csv but it didn't work. Can you help me?
Try to add it in
Or you can add it in your theme locale translate file as well
Not all lines can be translated, check the source code to see if it's plain text or wrapped like this:
<?php echo $this->__('YOUR TEXT GOES HERE') ?>
Plain text can't be translated automatically through Magento, you have to wrap the text like above for the translation to work.