PHP code:

$mediums = array('Wood', 'Paint');
$locations = array('Paris', 'London');

$args = array(
    'role' => 'artist',
    'orderby' => 'meta_value',
    'order' => 'ASC',
    'meta_query' => build_WP_Meta_Query_for_ACF_Checkbox(array(
        'relation' => 'AND',
        array(
            'artist_medium' => $mediums,
            'relation' => 'OR'
        ),
        array(
            'studio_region_location' => $locations,
            'relation' => 'OR'
        )
    ))
);
$user_query = new WP_USER_QUERY($args);

// TODO: appropriate error-handling! NO WARRANTY EXPRESSED OR IMPLIED, etc.
function build_WP_Meta_Query_for_ACF_Checkbox($criteria) {
    $result = array();
    // default relation should be 'AND', same as WP_Meta_Query.
    $result['relation'] = (isset($criteria['relation']) && strtoupper($criteria['relation']) === 'OR' ? 'OR' : 'AND');

    foreach ($criteria as $key => $value) {
        if (is_array($value)) {
            // "root" items have no named key, but are just array() wrappers for sub-query parts.
            if (!is_string($key)) {
                $sub_query = build_WP_Meta_Query_for_ACF_Checkbox($value);
                if (count($sub_query) > 0) {
                    $result[] = $sub_query;
                }
            } elseif (count($value) > 0) {
                // build up WP_Meta_Query args.
                foreach ($value as $val) {
                    $result[] = array(
                       'key' => $key,
                       // since the LIKE clause is querying a serialized array, we should
                       // enclose the search value in double quotes (at minimum.)
                       'value'=> '"' . $val . '"',
                       'compare' => 'LIKE'
                    );
                }
            }
        }
    }

    // our values for count() reflect that $result['relation'] is always set.
    if (count($result) <= 1) {
        // a checkbox field name was given, but had no search values.
        $result = array();
    } elseif (count($result) === 2) {
        // a checkbox field was provided with only one search value, and a relation is unnecessary.
        $result = $result[0];
    }
    return $result;
}