Use the wp_insert_post
hook to run when a post is saved:
function wpse_188435_save_movie_data( $post_id ) {
if ( $imdb = get_field( 'imdb_id', $post_id ) ) {
if ( $imdbInfo = get_imdb_connector_movie( $imdb ) ) {
if ( isset( $imdbInfo['actors'] ) )
update_post_meta( $post_id, 'actors', $imdbInfo['actors'] );
if ( isset( $imdbInfo['languages'] ) )
update_post_meta( $post_id, 'languages', $imdbInfo['languages'] );
}
}
}
add_action( 'wp_insert_post', 'wpse_188435_save_movie_data', 50 /* Late priority, will ensure ACF has saved all data first */ );