Very interesting observation.
Please refer to Seciton 6, 1 of the RTI Act:
<CENTER><TABLE cellSpacing=0 cellPadding=7 width=463 border=1><TBODY><TR><TD vAlign=top height=63>6. (1) A person, who desires to obtain any information under this Act, shall make a request in writing or through electronic means in English or Hindi or in the official language of the area in which the application is being made, accompanying such fee as may be prescribed, to—
Application can be made in English , Hindi or the regional language of the local area. Logically it should be replied to in the same language as the original application.
There are also some decisions of the CIC wherein the applicant applied in English but requested that the reply be given in Hindi. CIC has upheld this request.
However, pertinent point is what if the applicant uses a language that is not the "local" regional language. I think under Section 7, 3, a) you can ask for the requisite fees for doing getting the translation.