If participants cannot understand what the speakers say, have you wasted your time and money organising the conference?

You probably have wasted your money, unfortunately. All the participants at an international conference are likely to understand or to speak a certain amount of English, but perhaps not sufficiently to understand all the nuances of the presentations. That is why they often prefer to listen to the presentations in their own language and it makes it worthwhile for you to hire a professional interpreting service.