function hotelier_custom_currency( $currencies ) {
    $currencies[ 'IDR' ] = 'Indonesian Rupiah';

    return $currencies;
}
add_filter( 'hotelier_currencies', 'hotelier_custom_currency' );

function hotelier_custom_currency_symbol( $currency_symbol ) {
    $currency_symbol = 'Rp';

    return $currency_symbol;
}
add_filter( 'hotelier_currency_symbol', 'hotelier_custom_currency_symbol' );