plea

if the accused pleads guilty magistrates shall record the plea as nearly as possible in the words used by the accused and may in his discretion convict him thereon 

Yes,it is right preactice to so file an Affidavit detailing the reasons in a clear and coherent fashion.

Neither the plea recorded by the magistrate is in the language know to the accused nor there is any affidavit. If it is just based on a simple application in the language not known to the accused, is it a valid evidence in the other related case? It appears that the state-police forced the accused to plead guilty so that they could get benifit in the other case etc.

if it is not recorded in the language not known to accused u can prefer an appeal against the conviction order passed by the magistrate. it would be the good ground for u to prefer an appeal and to win the case. but it is the discretion of the sessions judge to remand the matter to the lower court for fresh trial or enquiry and once again trial has to take place before the same magistrate against whose order u preferred an appeal to the court of sessions.

if the plea recorded by the magistrate remains unchallenged certainly certainly it becomes valid evidence in other cases and it is valid till the disposal of the appeal preferred by u in the higher court. when appeal is pending against that order if the copy of the said order is used in other cases u can bring it to the notice of the court that the order has been stayed by the appellate court or the appeal is pending and so that u can avoid that order being used as evidence in other cases.

so prefer an appeal against the order passed on recording the plea and get it stayed immediately.