How to Modify Default Text in a Custom Taxonomy Admin Panel?

Checking the file /wp-admin/edit-tags.php, we see that it’s using the function _ex() to translate the string Description.

Following _ex path, we find that it finally uses the filter gettext_with_context to translate the text. So, it is possible to hook into this and modify the desired string.

The following code is a simplified version of @toscho’s Retranslate plugin. Please, check the original for a much more versatile code.

Use as a plugin or simply copy the code into your theme’s functions.php file.
In this example, we have an extra context and replacement.
To solve the OP Question, remove the second array item from both.

Configuration values in $params_71992:

• context of the string, as seen in /wp-admin/edit-tags.php
• replacements, strings for original and desired translation
• taxonomy, name of the taxonomy, category, post_tags or custom-taxonomy

<?php
/*
Plugin Name:    Translate Taxonomy Admin Strings
Plugin URI:     https://wordpress.stackexchange.com/q/71992/12615
Description:    Translate specific strings in admin screens Edit Tag
Version:        0.1
Author:         Rodolfo Buaiz
License:        GPL v2
*/


/** 
    Configuration 
    @params_71992['context']        as defined in /wp-admin/edit-tags.php
    @params_71992['replacements']   original text and translation
    @params_71992['taxonomy']       name of the taxonomy (category, post_tag, custom_taxonomy)
 */
$params_71992 = array (
        'context'       => array (
                'Taxonomy Description'
            ,   'Taxonomy Name'
        )
    ,   'replacements'  => array ( 
                'Description'   => 'Title'
            ,   'Name'          => 'The Name'
        )
    ,   'taxonomy'      => 'category'
);

// Run only in Edit Tags screens
add_action( 'admin_head-edit-tags.php', 'wpse_71992_register_filter' );

function wpse_71992_register_filter() 
{
    add_filter( 'gettext_with_context', 'wpse_71992_translate', 10, 4 );
}

function wpse_71992_translate( $translated, $original, $context, $domain ) 
{
    global $params_71992;

    // If not our taxonomy, exit early
    if( $params_71992['taxonomy'] !== $_GET['taxonomy'] )
        return $translated;

    // Text is not from WordPress, exit early
    if ( 'default' !== $domain ) 
        return $translated;

    // Check desired contexts
    if( !in_array( $context, $params_71992['context'] ) )
        return $translated;

    // Finally replace
    return strtr( $original, $params_71992['replacements'] );
}