Simple Global Attribute Size Switcher for WooCommerce Variable Products

gistfile1.php

<?php

/**
 * Show the size switcher.
 */
function size_switcher_display() {
	global $product;

	echo '<div class="wc-size-switcher">';
	echo __( 'Display sizes as:', 'woocommerce-size-switcher' ) . ' ';
	echo '<a href="' . esc_url( add_query_arg( 'size_display', 'UK' ) ) . '">UK</a> ';
	echo '<a href="' . esc_url( add_query_arg( 'size_display', 'EU' ) ) . '">EU</a> ';
	echo '<a href="' . esc_url( add_query_arg( 'size_display', 'US' ) ) . '">US</a> ';
	echo '</div>';
}
add_action( 'woocommerce_before_add_to_cart_form', 'size_switcher_display' );

/**
 * Switch size on demand.
 */
function switch_size() {
	if ( isset( $_GET['size_display'] ) && in_array( $_GET['size_display'], array( 'UK', 'EU', 'US' ) ) ) {
		wc_setcookie( 'size_display', sanitize_text_field( $_GET['size_display'] ) );
		wp_safe_redirect( wp_get_referer() );
		exit;
	}
}
add_action( 'wp', 'switch_size' );

/**
 * Convert UK size to X size.
 */
function get_converted_size( $size, $target ) {
	if ( 'US' === $target ) {
		// US sizes are just 1 more.
		$size ++;
	} elseif ( 'EU' == $target ) {
		// EU sizes are silly
		switch ( $size ) {
			case '6' :
				return '40';
			break;
			case '6.5' :
				return '40.5';
			break;
			case '7' :
				return '41';
			break;
		}
	}
	return $size;
}

/**
 * Change size when set. Terms are for example, UK 7, UK 6.5 etc.
 */
function get_term_switch_size( $_term, $taxonomy ) {
	// Change when cookie is set, and not the default (UK can be left alone as that is the original).
	if ( strstr( $_term->name, 'UK ' ) && isset( $_COOKIE['size_display'] ) && in_array( $_COOKIE['size_display'], array( 'EU', 'US' ) ) ) {
		$term_name = explode( ' ', $_term->name );
		$new_size  = get_converted_size( $term_name[1], $_COOKIE['size_display'] );

		// Change only if converted value was found.
		if ( $new_size !== $term_name[1] ) {
			$term_name[1] = $new_size;
			$term_name[0] = $_COOKIE['size_display'];
			$_term->name  = implode( ' ', $term_name );
		}
	}
	return $_term;
}
add_filter( 'get_term', 'get_term_switch_size', 10, 2 );