Dear Isha,
If your customer service staff are unable to solve the customer query then it could be because of the lack of the product knowledge or technical knowledge. It need not be because of communication skills.
The second reason for their inability to solve the customers' queries is because of telling them to talk in English or some specific language. Therefore, in addition to lack of product knowledge they might be lacking command over English or the language in which customers' talk. Therefore, it could a problem of wrong recruitment and employee training may not solve it.
As far as customers' queries are concerned, have you done analysis or categorisation of the queries? On what issues customers ask the queries most and on what issues the queries are least? Do you know that? Irrespective of the type of query, your training should be on the sequence of the questions that customer service personnel should ask to solve their queries.
Thanks,
Dinesh Divekar