// just pass in the attribute and the attribute slug
// and the return value is the attribute's name

// example : (assuming attribute size has the option of "Extra Small" withe the slug of "extra-small")
// echo attribute_slug_to_title('attribute_pa_size', 'extra-small');
// returns
// "Extra Small"

// code reworked from woocommerce/classes/class-wc-cart.php

// attribute slug to title
if ( ! function_exists( 'attribute_slug_to_title' ) ) {
	function attribute_slug_to_title( $attribute ,$slug ) {
		global $woocommerce;
		if ( taxonomy_exists( esc_attr( str_replace( 'attribute_', '', $attribute ) ) ) ) {
			$term = get_term_by( 'slug', $slug, esc_attr( str_replace( 'attribute_', '', $attribute ) ) );
			if ( ! is_wp_error( $term ) && $term->name )
				$value = $term->name;
		} else {
			$value = apply_filters( 'woocommerce_variation_option_name', $value );
		}
		return $value;
	}
}