Description 
 If the path is not given then it will be the root of the plugin directory.
 The .mo file should be named based on the text domain with a dash, and then the locale exactly.
    Parameters 
  - $domain
-   (string) (Required) Unique identifier for retrieving translated strings  
- $deprecated
-   (string|false) (Optional)  Deprecated. Use the $plugin_rel_path parameter instead.   Default value: false 
- $plugin_rel_path
-   (string|false) (Optional)  Relative path to WP_PLUGIN_DIR where the .mo file resides.   Default value: false 
Return 
 (bool) True when textdomain is successfully loaded, false otherwise.
    Source 
  File: wp-includes/l10n.php 
 function load_plugin_textdomain( $domain, $deprecated = false, $plugin_rel_path = false ) {
	/**
	 * Filters a plugin's locale.
	 *
	 * @since 3.0.0
	 *
	 * @param string $locale The plugin's current locale.
	 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
	 */
	$locale = apply_filters( 'plugin_locale', determine_locale(), $domain );
	$mofile = $domain . '-' . $locale . '.mo';
	// Try to load from the languages directory first.
	if ( load_textdomain( $domain, WP_LANG_DIR . '/plugins/' . $mofile ) ) {
		return true;
	}
	if ( false !== $plugin_rel_path ) {
		$path = WP_PLUGIN_DIR . '/' . trim( $plugin_rel_path, '/' );
	} elseif ( false !== $deprecated ) {
		_deprecated_argument( __FUNCTION__, '2.7.0' );
		$path = ABSPATH . trim( $deprecated, '/' );
	} else {
		$path = WP_PLUGIN_DIR;
	}
	return load_textdomain( $domain, $path . '/' . $mofile );
}       Changelog 
     | Version | Description | 
    | 4.6.0 | The function now tries to load the .mo file from the languages directory first. | 
  | 1.5.0 | Introduced. |