If you have in your /wp-content/plugins/ directory translation file for language you search way to add to Plugins Language Switcher drop down list, Plugins Language Switcher will show that language in the language selection drop down list automatically.
Place the whole WordPress back-end translation additional language .mo file to wp-content/languages to get full translation of administrator back-end to needed language. Plugins settings pages could be translated if they have its own translation files.
You can find WordPress translation file for your language at the translation repository at http://svn.automattic.com/wordpress-i18n/
Requires: 3.0 or higher Compatible up to: 3.8.1 Last Updated: 2013-12-24 Downloads: 50,367