The string is not internationalized (not translatable).
The correct way is :
return $output . 'ID) . '">' . __('Read More...', 'theme_domain');You should replace 'theme_domain' by the correct string for your theme (look for the function load_theme_texdomain in your theme files to find the right name of your theme text domain)
Then use the plugin codestyling localization to translate it.
I recommend doing a child theme to avoid overwriting of your modification at next update. And also submit the correction to the theme author.