functions.php

<?php

/*
 * Show posts assigned to 'International (intl)' Edition
 * in all editions.
 */
function wtnerd_better_editions( $query ) {
	if ( $query->is_category() && $query->is_main_query() ) {
		$query->set( 'post_type', array( 'post' ) );

		// Get current tax query
		$tax_query = $query->get( 'tax_query' );

		$tax_query['relation'] = 'OR';

		$tax_query[] = array(
			'taxonomy' => 'category',
			'field'    => 'slug',
			'terms'    => 'intl',
			'operator' => 'IN'
		);

		$query->set( 'tax_query', $tax_query );
	}
	return $query;
}
add_action( 'pre_get_posts', 'wtnerd_better_editions' );