Writing Bangla has been a lot of easier since the latest versions of ubuntu has IBus built-in inside it.
You just have to paste the following commands on the terminal.
01. Press Ctrl+Alt+T to open Terminal.
02. paste the following commands in terminal and provide the password.
sudo apt-get install ibus-m17n m17n-db m17n-contrib ibus-gtk
sudo dpkg -L m17n-db|grep bn
sudo dpkg -L m17n-contrib|grep bn
Of course you need an Internet connection to complete the installation process.
If ibus is not installed on your system, install it by providing the following commands on terminal.
sudo apt-get install ibus
then follow the instructions on this link.