Twenty_Twelve category.php

<?php
/**
 * The modified template for displaying Category pages.
 *
 * Display original if translation does not exist.
 *
 * Used to display archive-type pages for posts in a category.
 *
 * @link http://codex.wordpress.org/Template_Hierarchy
 *
 * @package WordPress
 * @subpackage Twenty_Twelve
 * @since Twenty Twelve 1.0
 */

get_header();

// WPML support
global $sitepress;
// save current language
$current_lang = $sitepress->get_current_language();
//get the default language
$default_lang = $sitepress->get_default_language();
//fetch posts in default language
$sitepress->switch_lang($default_lang);
$cat = get_query_var('cat');
//query args
$custom_query_args = array(
    'cat' => icl_object_id($cat, 'category', FALSE,'en')
);

//build query
$custom_query = new wp_query($custom_query_args);
//\ WPML support
?>

	<section id="primary" class="site-content">
		<div id="content" role="main">

		<?php
// WPML support
		if ( $custom_query->have_posts() ) :
//\ WPML support
		?>
			<header class="archive-header">
				<h1 class="archive-title"><?php printf( __( 'Category Archives: %s', 'twentytwelve' ), '<span>' . single_cat_title( '', false ) . '</span>' ); ?></h1>

			<?php if ( category_description() ) : // Show an optional category description ?>
				<div class="archive-meta"><?php echo category_description(); ?></div>
			<?php endif; ?>
			</header><!-- .archive-header -->

			<?php
			/* Start the Loop */
// WPML support
			while ( $custom_query->have_posts() ) : $custom_query->the_post();
				//check if a translation exist
				$t_post_id = icl_object_id($post->ID, 'post', false, $current_lang);
				if(!is_null($t_post_id)){
					// get the post object
					$t_post = get_post( $t_post_id);
					// include the output code
					include (get_stylesheet_directory() . '/content-t.php');
				}
				//no translation? display default language
				else{
					/* Include the post format-specific template for the content. If you want to
					 * this in a child theme then include a file called called content-___.php
					 * (where ___ is the post format) and that will be used instead.
					 */
					get_template_part( 'content', get_post_format() );
				}
			endwhile;
			wp_reset_query();
			$sitepress->switch_lang($current_lang);
//\ WPML support

			twentytwelve_content_nav( 'nav-below' );
			?>

		<?php else : ?>
			<?php get_template_part( 'content', 'none' ); ?>
		<?php endif; ?>

		</div><!-- #content -->
	</section><!-- #primary -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>