global class SalesOrder {
    global String name; //contains cyrillic letters
    ...
}

@HttpGet
global static SalesOrderWrapper getSalesOrder() {
    ...
    return salesOrder;
}

@HttpGet
global static void getSalesOrder() {
    ...
    RestContext.response.responseBody = Blob.valueOf(JSON.serialize(orderWrapper));
}

response.headers.put('Content-Type', 'application/json');
response.headers.put('charset', 'UTF-8');