Yes, Samsung Metro XL does support the Marathi language.
You can use Facebook which is pre-installed on this phone but you can not use WhatsApp in this phone.
Yes, Samsung Metro XL does have flash with camera.
You can store up to 1000 contacts in Metro XL. In addition, you can store up to 250 contacts in each of your sim card. So, if you are using 2 sim cards, in total you can store up to 1500 contacts.
Yes, Metro XL comes with pre-installed Facebook App.