No, It's not mandatory. You need to be fluent in English and Hindi. A foreign language opens up the door for international airlines and foreign airlines. you can pursue diploma courses in Air Hostess training or cabin crew and hospitality course from any reputed institutes. You can also do B.Sc. in Aviation, Hospitality and Tourism. Educational qualification for a cabin crew is 12th. 

Requirements to be a cabin crew :
Fluent English
Passionate for delivering excellent service on every occasion
Being trustworthy and reliable
Being correct and precise in their job
Proactively seek solutions and take personal responsibility for resolving problems
Be willing and able to work shifts
Minimum age of 18 or 21, depending on the airline
Minimum height of 158cm or 160cm, depending on the airline
Being able to swim
No visible tattoos or body piercings while wearing the uniform
No criminal record
Right to live and work in that country you will be based at or being able to get a working visa in that country
Have a healthy BMI
Minimum high school education
Have a valid Passport
DGCA (the Indian regulatory authority which is responsible for regulatory and safety oversight of Indian aviation, and a lot more), specifies that Cabin Crew need to be trained on the Specific type of aircraft they work on. This is called Safety and Emergency Procedures(SEP) training and is specific to a particular type of aircraft(For example, Cabin Crew with SEP card from Go Air can join Indigo without a full training again because both the airlines operate the same type of aircraft-A320 but crew with a SEP card on ATR-72/Q-400 of Jet/Spice Jet cant have to get trained afresh on A320 if joining Indigo or Go Air by that airline), this training deals with crew understanding of aircraft, response in case of emergency etc, which is the primary duty of cabin crew.
A SEP card is issued to the crew after they pass the exams after this training only after which they can fly.
You can do courses from:
Air Hostess Academy (AHA), Bangalore
Lovely Professional University, Punjab 

Hey Raj.

you dont need any foreign language certificate to apply for air hostess form.

you need to be fluent in any foreign language,that gives you an edge over others. So it will be an advantage but no certificate is reqired.

if you liked the answer kindly upvote. For any further queries feel free to revert back.

goodluck.