When I get a request to send a proposal to a conference, I go to the website and have a look at the previous edition to get an idea of how was it, what topics were discussed, who went there, etc.
If the website is in a language I don't understand, I feel extremely discouraged to send anything at all. This tells me that the organisers are not interested in international visitors, and that if I have an issue or a question I will have serious difficulties making myself understood.
I do not want to be in this position, hence I won't send a proposal.
PS: No, using online translation services is not the solution.