Most of the feed should use the language your WordPress site is configured for. See Installing WordPress in Your Language.
There are some items, like the “Facebook Status” box at top of the feed and the “comments” link, that are in English. However, you can translate the plugin using the “default.po” file found in the “wp-content/plugins/jsl3-facebook-wall-feed/languages” directory. Use Poedit to translate the plugin into your language and then save the PO file using the text domain (jsl3-fwf), language code and country code as the name. For example, if you translate the plugin into German, you should save the file as “jsl3-fwf-de_DE.po”. Finally, place the translated PO file and its corresponding MO file in the “wp-content/plugins/jsl3-facebook-wall-feed/languages” directory.
If you do create a German PO file, give me a link to your site and I will include that translation in future versions of the plugin.