While waiting for the plugin author to update the plugin so this value gets properly translated, I went ahead and edited the code for my own use. You can do the same thing. Edit the file “yawpp/display_yawpp.php”
You can do this by clicking on ‘plugins’ -> ‘editor’ and then selecting /YAWPP (Yet Another WordPress Plugin Petition)’ from the select box at the top right of screen, press the ‘select’ button then click on ‘yawpp/display_yawpp.php’ from the list that appears along the right edge of the screen. In the main edit area scroll down until you find where it says: “<input type=”submit” value=”Valider” name=”submit_yawpp” />” (you can use your browser’s ‘find’ feature to find the word valider) and replace the word “Valider” with whatever you want (I changed it to “Sign Now”).