This is funny thought. This issue has been around for quite awhile now. Matt’s said ([wp-hackers] Decision time in re: admin rework) they wont change the “Howdy” to “hello” or “G’day” as its a localization specific (l10n). Thats true. But If you think proper greeting is important, you can try the following steps.

Wordpress admin-header.php

• Open */wp-admin/admin-header.php
• Login to WordPress Admin goto Manage > Files > Type in “wp-admin/admin-header.php”
  
• find on line 46

  <div id="user_info"><p><?php printf(__('Howdy, <strong>%s</strong>.'), $user_identity) ?> [<a href="<?php echo get_option('siteurl'); ?>/wp-login.php?action=logout" title="<?php _e('Log out of this account') ?>"><?php _e('Sign Out'); ?></a>, <a href="profile.php"><?php _e('My Profile'); ?></a>] </p></div>
  

• Replace with

  <div id="user_info"><p><?php echo __('Hello').', <strong>' $user_identity.'</strong>'; ?> [<a href="<?php echo get_option('siteurl'); ?>/wp-login.php?action=logout" title="<?php _e('Log out of this account') ?>"><?php _e('Sign Out'); ?></a>, <a href="profile.php"><?php _e('My Profile'); ?></a>] </p></div>
  

g’day :p

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