This extension load the english version of a language then it will replace lines by the existing ones on the new requested langauge.

MY_Lang.php

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

/**
 * MY_Lang
 *
 * This extension of CodeIgniter CI_Lang allows you to load any file in any
 * language even if the language file does not exists BUT the english version
 * must exist in order to proceed.
 *
 * So to make short: ALWAYS have the english version of the language file then
 * create as many languages versions as you wish. If the line does not exist
 * in the requested language, it will user the english one.
 *
 * @package 	CodeIgniter
 * @category 	Core
 * @author 	Kader Bouyakoub <[email protected]>
 * @link 	http://www.bkader.com/
 */

class MY_Lang extends CI_Lang
{
	/**
	 * Holds Config class object
	 * @var array
	 */
	protected $config;

	/**
	 * Constructor
	 * @param 	none
	 * @return 	void
	 */
	public function __construct()
	{
		parent::__construct();
	}

    // ------------------------------------------------------------------------

    /**
     * Does the same job as parent's load method except that it will load the
     * english version first, then it will replace values with those found
     * inside requested language file.
     *
     * @param   mixed   $langfile   Language file name
     * @param   string  $idiom      Language name (english, etc.)
     * @param   bool    $return     Whether to return the loaded array of translations
     * @param   bool    $add_suffix Whether to add suffix to $langfile
     * @param   string  $alt_path   Alternative path to look for the language file
     *
     * @return  void|string[]   Array containing translations, if $return is set to TRUE
     */
    public function load($langfile, $idiom = '', $return = FALSE, $add_suffix = TRUE, $alt_path = '')
    {
        // In case of multiple files
        if (is_array($langfile))
        {
            foreach ($langfile as $value)
            {
                $this->load($value, $idiom, $return, $add_suffix, $alt_path);
            }

            return;
        }

        // Remove .php extension if set
        $langfile = str_replace('.php', '', $langfile);

        // Remove the _lang suffix
        if ($add_suffix === TRUE) {
            $langfile = preg_replace('/_lang$/', '', $langfile).'_lang';
        }

        $langfile .= '.php';

        if (empty($idiom) OR ! preg_match('/^[a-z_-]+$/i', $idiom))
        {
            $idiom = $this->config->item('language');
        }

        if ($return === FALSE && isset($this->is_loaded[$langfile]) && $this->is_loaded[$langfile] === $idiom)
        {
            return;
        }

        // Load the english version first, then we load the the requested one
        $full_lang = array();

        // Load the base file, so any others found can override it
        $basepath = BASEPATH.'language/english/'.$langfile;
        if (($found = file_exists($basepath)) === TRUE)
        {
            include($basepath);
        }

        // Do we have an alternative path to look in?
        if ($alt_path !== '')
        {
            $alt_path .= 'language/english/'.$langfile;
            if (file_exists($alt_path))
            {
                include($alt_path);
                $found = TRUE;
            }
        }
        else
        {
            foreach (get_instance()->load->get_package_paths(TRUE) as $package_path)
            {
                $package_path .= 'language/english/'.$langfile;
                if ($basepath !== $package_path && file_exists($package_path))
                {
                    include($package_path);
                    $found = TRUE;
                    break;
                }
            }
        }

        if ($found !== TRUE)
        {
            show_error('Unable to load the requested language file: language/english/'.$langfile);
        }

        $full_lang = isset($lang) ? $lang : array();
        $lang = array();

        // Load the base file, so any others found can override it
        $basepath = BASEPATH.'language/'.$idiom.'/'.$langfile;
        if (file_exists($basepath))
        {
            include($basepath);
        }

        // Do we have an alternative path to look in?
        if ($alt_path !== '')
        {
            $alt_path .= 'language/'.$idiom.'/'.$langfile;
            if (file_exists($alt_path))
            {
                include($alt_path);
            }
        }
        else
        {
            foreach (get_instance()->load->get_package_paths(TRUE) as $package_path)
            {
                $package_path .= 'language/'.$idiom.'/'.$langfile;
                if ($basepath !== $package_path && file_exists($package_path))
                {
                    include($package_path);
                    break;
                }
            }
        }

        if ($found !== TRUE)
        {
            show_error('Unable to load the requested language file: language/'.$idiom.'/'.$langfile);
        }

        isset($lang) OR $lang = array();

        $full_lang = array_replace_recursive($full_lang, $lang);

        if ( ! isset($full_lang) OR ! is_array($full_lang))
        {
            log_message('error', 'Language file contains no data: language/'.$idiom.'/'.$langfile);

            if ($return === TRUE)
            {
                return array();
            }
            return;
        }

        if ($return === TRUE)
        {
            return $full_lang;
        }

        $this->is_loaded[$langfile] = $idiom;
        $this->language = array_merge($this->language, $full_lang);
        unset($full_lang);

        log_message('info', 'Language file loaded: language/'.$idiom.'/'.$langfile);
        return TRUE;
    }
}

/* End of file MY_Lang.php */
/* Location: ./application/core/MY_Lang.php */