Well.. After reading Rcalleja answer .. You can use the plugin provided , but there is really no need for it code hackings or php juggling ..

All you have to do is to use the quicktags in the attachments fields :

[:en]English Description[:fr]French description[:de]German description

After that ( and now it depends on the access you have to theme files, but if you are here - I assume it is not a problem ) you will need to use a small built in function called qtrans_use() which tells qtranslate which language to use ..

qtrans_use('fr',$string,false);

so for example, if you are using the very popular function the_post_thumbnail() , you will have to translate itßs attributes . since it is using wp_get_attachment_image, we know the attributes are for example (from codex ) :

$default_attr = array(
    'src'   => $src,
    'class' => "attachment-$size",
    'alt'   => trim(strip_tags( $wp_postmeta->_wp_attachment_image_alt )),
);

Now , we need to get the alt and title like so :

$thumb_id = get_post_thumbnail_id(get_the_ID());
$alt = get_post_meta($thumb_id, '_wp_attachment_image_alt', true);
$title = get_post_meta($thumb_id, '_wp_attachment_image_title', true);

So putting 2 and 2 together :

    $attr = array(
'alt'   => qtrans_use('de',$alt,false),
'title' => qtrans_use('de',$alt,false),
    );

and then

the_post_thumbnail('my_size',$attr);

et voila ! the title and description will be in german .. ( 'de' )

( the 'de' is just an example, to automate the process one would another built-in qtrans function - qtrans_getLanguage() )

I have used this method in the past , and it always worked for me . In fact, it is possible to translate any and all aspects of wordpress with qtranslate , without the needs for extra plugins ..

qtranlaste is one of the best written plugins ( albeit a bit old.. ) and I myself am a big fan of it . The author has used internal filters and hooks long before most of the current "experts" have started to apply them in plugins or even knew existed. in fact, It is ( IMHO ) the best translation plugin .

The problem is that ( like wordpress itself ) it is very extensive and extendable , but it has a lot of filter and functions that are poorly documented , which require to "dig into the code " .

Luckily , someone has already done so .

Good question .

Well.. After reading Rcalleja answer .. You can use the plugin provided , but there is really no need for it, nor for code hacking or php juggling ..

All you have to do is to use the quicktags in the attachments fields :

[:en]English Description[:fr]French description[:de]German description

After that ( and now it depends on the access you have to theme files, but if you are here - I assume it is not a problem ) you will need to use a small built in function called qtrans_use() which tells qtranslate which language to use ..

qtrans_use('fr',$string,false);

so for example, if you are using the very popular function the_post_thumbnail() , you will have to translate itßs attributes . since it is using wp_get_attachment_image, we know the attributes are for example (from codex ) :

$default_attr = array(
    'src'   => $src,
    'class' => "attachment-$size",
    'alt'   => trim(strip_tags( $wp_postmeta->_wp_attachment_image_alt )),
);

Now , we need to get the alt and title like so :

$thumb_id = get_post_thumbnail_id(get_the_ID());
$alt = get_post_meta($thumb_id, '_wp_attachment_image_alt', true);
$title = get_post_meta($thumb_id, '_wp_attachment_image_title', true);

So putting 2 and 2 together :

    $attr = array(
'alt'   => qtrans_use('de',$alt,false),
'title' => qtrans_use('de',$alt,false),
    );

and then

the_post_thumbnail('my_size',$attr);

et voila ! the title and description will be in german .. ( 'de' )

( the 'de' is just an example, to automate the process one would another built-in qtrans function - qtrans_getLanguage() )

I have used this method in the past , and it always worked for me . In fact, it is possible to translate any and all aspects of wordpress with qtranslate , without the needs for extra plugins ..

qtranlaste is one of the best written plugins ( albeit a bit old.. ) and I myself am a big fan of it . The author has used internal filters and hooks long before most of the current "experts" have started to apply them in plugins or even knew existed. in fact, It is ( IMHO ) the best translation plugin .

The problem is that ( like wordpress itself ) it is very extensive and extendable , but it has a lot of filter and functions that are poorly documented , which require to "dig into the code " .

Luckily , someone has already done so .

EDIT I :

There is also another method to filter the whole post thumbnail produced HTML using the post_thumbnail_html filter. But since the above method is general and can be applied on any element ( not just post thumbnail ) , and since you did not specifically inquired for post thumbs but media in general - I thought it is preferred to explain it rather then a much more specific one .