In the NGFB Buttons settings, you can select a Default Language for the Facebook, Google+, and the Twitter social sharing buttons. The Free version of NGFB will always display the sharing buttons in that language. The Pro version includes support for the WordPress locale setting, which means that as the webpage changes language, so will the buttons. The Default Language setting then becomes a fallback in case a matching language cannot be found.
If you have the Pro version, and the sharing buttons do not show the correct language, your WordPress locale setting might be set incorrectly. If you're using a multilingual plugin to manage languages / translations, make sure the plugin uses a four letter language code like "en_US" instead of a simple two letter code like "en" (most do, so this shouldn't be a problem). NGFB will try to find a logical match for two letter language codes, but this may not always be successful.
Not all languages are supported by all buttons. If NGFB cannot find a matching language for the WordPress locale, it will use the Default Language you've selected.
If the social sharing buttons still do not appear in the correct language, you can try defining a WPLANG constant in your wp-config.php file (if one hasn't already been defined). For example:
As a last resort, you can disable support for the WordPress locale on the NGFB Advanced settings page.