How can I keep the current language prefix for internal link?
Generally you want to use Drupal core functions as much as possible. Both the l() and url() functions fit perfectly your need. The difference between them is that l() returns an anchor tag, and url() returns a URL without HTML.

As a perk, the l() function allows you to specify the language for the link. Example:

&lt;?php
$options = array('language' =&gt; 'jp');
$link = l('About Us', 'about-us', $options);