MW_EXT_Kernel.class.php
public static function getMessageText( $id, $key ) {
		$string  = 'mw-ext-' . $id . '-' . $key;
		$message = wfMessage( $string )->inContentLanguage();
		$out     = $message->text();

		return $out;
	}
public static function getMessageText( $id, $key ) {
		$string  = 'mw-ext-' . $id . '-' . $key;
		$message = wfMessage( $string )->inContentLanguage();
		$out     = $message->text();

		return $out;
	}
Wiki Framework: Message.

@param $id
@param $key

@return string