You can translate the plugin from translate.wordpress.org site. Link below:
Alternatively, You can use the PO File editor to do the translation. Here is a step by step guide on how you can translate the WordPress Simple Shopping Cart plugin in your language.
1. Download and install Poedit (http://www.poedit.net/download.php)
2. Get the plugins POT file (wordpress-simple-paypal-
3. Run Poedit software (if it is not running already)
4. Open the POT file in Poedit. (Click the open link in the software then browse to the POT file location and select that file)
5. Select each translatable text and add the translation in the box that says “Translation”
6. Finish translating the whole file.
7. Go to File -> Save as to save your translations in a PO file.
8. When you are finished translating, go to File -> Save as again to generate the MO file.
Or you can set your Poedit to always compile a MO file when saving changes by clicking File -> Preferences and on the Editor tab check the Automatically compile .mo file on save box.
9. Contact us and send us the translated .mo and .po files and we will add it to the plugin.