Clerical vacancies in banks, the selection and filling are usually State-wise. Such posts necessitate frontline jobs needing direct contact and communication with people. Hence it may require proficiency or at least working knowledge in that local language. That is why they put the condition of knowledge of local language. Moreover, the applications, chalans, and sometimes even cheques are filled in local language. So, most clerical jobs require knowledge in local language.
Sometimes candidates just send their application preferring a particular state when the vacancies are more than other places. In major metropolitan cities and major urban centres one can manage with knowledge of English and Hindi and picking local language by experience. This is not possible when the posting is in interior villages or small towns.
In your case you say it happened by an inadvertent ticking on the slot for local language.
If you are already selected for posting then just face the situation and go ahead.
Anyway don't panic. If it is a genuine mistake, you can tell about it when you are called for joining. Alternatively if you have sufficient time for joining, try to learn the language at least to the level of speaking and understanding.
You can also contact the relevant office that deals with posting. You can convince them and get their help in this regard. They may accommodate you in the offices or seats where you need not directly deal with public. They may help you to give a posting in a suitable place/office where you will be able to manage. For example they may post you in some back offices or Regional Offices etc where you are not needed contact or communicate with public. Then you can work hard to learn the language as early as possible. If needed you may also seek help of the employees union people who also may be able to help you.
Hence do not worry unnecessarily. However if you have any doubts or misapprehensions in mind, keep applying for other similar suitable jobs in the meantime. You will be getting what is destined to you. But be careful in future while filing up forms and furnishing bio- data. Give only facts and truths.