Unfortunately, you will need to root the phone because the font files are stored on the system partition and without a superuser you will not be able to access the location where the files are stored. After the phone has been rooted, place the font in the /system/fonts and you should have Malayalam available as a font selection.
It's known that installing a custom font will brick the device and then the only way to fix it is through Recovery and Factory reset. Be careful and create a backup before doing anything on the system partition.
