Samsung B310 comes with 800 mAh battery. But it is very efficient. You do not require to charge the phone for almost 3 days with moderate usage.
Yes, Samsung Guru Music 2 works with Kannada langauge.
Yes, you can create contacts groups in Samsung Guru Music 2.
This seems to be some software related issue. You should visit the nearest Samsung authorized service center and get it checked.
No. You cannot use 4g network sim cards. You need to select 2G network for your sim card and then you can use it.