It is a given, that most people would like to communicate using their own language, not only does this result in more accurate communication regarding what the customer wants. It also tends to reduce barriers that can arise from cultural differences thus leading to more open and honest communication and hopefully because of this positive experience an increase in trust between the business and the client.

Having established trust, your client should feel more comfortable when doing business with you. Multilingual customer support tends to increase loyalty to your product.  

Therefore, there is more likelihood of your client returning to your company to do more business. There is also the possibility that they will in turn recommend your company to their business associates with similar requirements. From my personal experience in providing multilingual support if your client has had a good experience in their own language, they are likely to return.