Cannot change language

Hello,
I’m using rating-widget v1.3.4.
From the plugin settings, “Rating-Widget Options” part, I change the language from “English( Default)” to “German – Deutsch” (or any other language).
When I click on the ‘Save’ button, the value of Language field get back to “English (Default)”…
However, I have write access on the folder “plugins/rating-widget”.
I made the change manually in the file ‘lib/config.php’ :
define(“WP_RW__DEFAULT_LNG”, “de”);
Now, the field appears as ‘German – Deutsch’ in the settings of the plugin, however the text on stars is still showing in english…
Moreover, if I make any changes in the “Rating-Widget Options” part, and click ‘Save’ => modifications are not saved and get back to the previous value…
If someone has any idea…
Thank you by advance.
Olivier.