Updated Greek translation.

framework/messages/el/app.php

+<?php
+/**
+ * Message translations.
+ *
+ * This file is automatically generated by 'yii message' command.
+ * It contains the localizable messages extracted from source code.
+ * You may modify this file by translating the extracted messages.
+ *
+ * Each array element represents the translation (value) of a message (key).
+ * If the value is empty, the message is considered as not translated.
+ * Messages that no longer need translation will have their translations
+ * enclosed between a pair of '@@' marks.
+ *
+ * Message string can be used with plural forms format. Check i18n section
+ * of the guide for details.
+ *
+ * NOTE: this file must be saved in UTF-8 encoding.
+ */
+return [
+    'Hello, {username}!' => 'Γεια σου, {username}!',
+];