Add loading of TGMPA native translation files.

Implemented in a way that the language loading will work independently of whether TGMPA is included in a plugin or a theme.
Including compatibility for looking in both the WP_LANG_DIR/plugins/ as well as the WP_LANG_DIR/themes/ directory for translations served by Wp.org.

Ignore warnings from PHPCS as we *really* should suppress warning on `is_readable()`.

class-tgm-plugin-activation.php

@@ -260,6 +260,15 @@ public function __construct() {
 			// Announce that the class is ready, and pass the object (for advanced use).
 			do_action_ref_array( 'tgmpa_init', array( $this ) );
 
+			/*
+			 * Load our text domain and allow for overloading the fall-back file.
+			 *
+			 * {@internal IMPORTANT! If this code changes, review the regex in the custom TGMPA
+			 * generator on the website.}}
+			 */
+			add_action( 'init', array( $this, 'load_textdomain' ), 5 );
+			add_filter( 'load_textdomain_mofile', array( $this, 'overload_textdomain_mofile' ), 10, 2 );
+
 			// When the rest of WP has loaded, kick-start the rest of the class.
 			add_action( 'init', array( $this, 'init' ) );
 		}
@@ -430,6 +439,90 @@ public function init() {
 			}
 		}
 
+		/**
+		 * Load translations.
+		 *
+		 * @since 2.x.x
+		 *
+		 * (@internal Uses `load_theme_textdomain()` rather than `load_plugin_textdomain()` to
+		 * get round the different ways of handling the path and deprecated notices being thrown
+		 * and such. For plugins, the actual file name will be corrected by a filter.}}
+		 *
+		 * {@internal IMPORTANT! If this function changes, review the regex in the custom TGMPA
+		 * generator on the website.}}
+		 */
+		public function load_textdomain() {
+			if ( is_textdomain_loaded( 'tgmpa' ) ) {
+				return;
+			}
+
+			if ( false !== strpos( __FILE__, WP_PLUGIN_DIR ) || false !== strpos( __FILE__, WPMU_PLUGIN_DIR ) ) {
+				// Plugin, we'll need to adjust the file name.
+				add_action( 'load_textdomain_mofile', array( $this, 'correct_plugin_mofile' ), 10, 2 );
+				load_theme_textdomain( 'tgmpa', dirname( __FILE__ ) . '/languages' );
+				remove_action( 'load_textdomain_mofile', array( $this, 'correct_plugin_mofile' ), 10 );
+			} else {
+				load_theme_textdomain( 'tgmpa', dirname( __FILE__ ) . '/languages' );
+			}
+		}
+
+		/**
+		 * Correct the .mo file name for (must-use) plugins.
+		 *
+		 * Themese use `/path/{locale}.mo` while plugins use `/path/{text-domain}-{locale}.mo`.
+		 *
+		 * {@internal IMPORTANT! If this function changes, review the regex in the custom TGMPA
+		 * generator on the website.}}
+		 *
+		 * @since 2.x.x
+		 *
+		 * @param string $mofile Full path to the target mofile.
+		 * @param string $domain The domain for which a language file is being loaded.
+		 * @return string $mofile
+		 */
+		public function correct_plugin_mofile( $mofile, $domain ) {
+			// Exit early if not our domain (just in case).
+			if ( 'tgmpa' !== $domain ) {
+				return $mofile;
+			}
+			return preg_replace( '`/([a-z]{2}_[A-Z]{2}.mo)$`', '/tgmpa-$1', $mofile );
+		}
+
+		/**
+		 * Potentially overload the fall-back translation file for the current language.
+		 *
+		 * WP, by default since WP 3.7, will load a local translation first and if none
+		 * can be found, will try and find a translation in the /wp-content/languages/ directory.
+		 * As this library is theme/plugin agnostic, translation files for TGMPA can exist both
+		 * in the WP_LANG_DIR /plugins/ subdirectory as well as in the /themes/ subdirectory.
+		 *
+		 * This method makes sure both directories are checked.
+		 *
+		 * {@internal IMPORTANT! If this function changes, review the regex in the custom TGMPA
+		 * generator on the website.}}
+		 *
+		 * @since 2.x.x
+		 *
+		 * @param string $mofile Full path to the target mofile.
+		 * @param string $domain The domain for which a language file is being loaded.
+		 * @return string $mofile
+		 */
+		public function overload_textdomain_mofile( $mofile, $domain ) {
+			// Exit early if not our domain, not a WP_LANG_DIR load or if the file exists and is readable.
+			if ( 'tgmpa' !== $domain || false === strpos( $mofile, WP_LANG_DIR ) || @is_readable( $mofile ) ) {
+				return $mofile;
+			}
+
+			// Current fallback file is not valid, let's try the alternative option.
+			if ( false !== strpos( $mofile, '/themes/' ) ) {
+				return str_replace( '/themes/', '/plugins/', $mofile );
+			} elseif ( false !== strpos( $mofile, '/plugins/' ) ) {
+				return str_replace( '/plugins/', '/themes/', $mofile );
+			} else {
+				return $mofile;
+			}
+		}
+
 		/**
 		 * Hook in plugin action link filters for the WP native plugins page.
 		 *

phpcs.xml

@@ -4,7 +4,7 @@
 	<file>class-tgm-plugin-activation.php</file>
 	<file>example.php</file>
 	<arg name="report" value="full"/>
-	<arg value="sp"/>
+	<arg value="spn"/>
 	<rule ref="WordPress">
 		<exclude name="WordPress.VIP" />
 	</rule>