1. It is hard to believe that your partner did not see or know what paper he is signing which was given to him by his mother?
2. You have stated that his mother had mentioned that "this paper is a registered marriage certificate...."
3. How can it be a registered marriage certificate unless he has married and registered his marriage before the local Registrar of Marriage?
4. Or was it an application before the Registrar of Marriage for getting his marriage registered after completion of one month of the stipulated notice period for getting a registered marriage?
5. If it is a marriage certificate then he has already married and registered his marriage and now applying for the VISA of his wife for taking her to Germany as his wife.
6. If he has already got married to her and registered his said marriage then all are over. he is a married man now and as per Indian law can not continue to have live in relationship with you.
7. If he has not yet married her and not got any marriage registered, then he can easily refuse to marry the said engaged girl on some acceptable ground and he will not be arrested ordinarily.
8. If the girl complaint with evidence and prove that he is refusing to marry her since she has refused to pay a sum of amount demanded by him for marrying her (in India this perverted system is prevalent in some greedy people who extort money from the bride's side) then only he will be arrested till he can manage to get bail or can avoid getting arrested if he can manage to get an anticipatory bail.
9. The only legal action the girl's family can take is that they can file a damage suit for damaging their reputation in their society, trust and also breach of assurance. They can also lodge/ file a dowry demand complaint/case as explained above.