Actually it was pretty easy. I changed the default greeter from
In that file I found “greeter session” and choose
between “lightdm-gtk-greeter” and “pi-greeter”, as the default greeter session. (You can choose your own greeter session).
Then I created a .po file and translated the strings I needed(using the "msgid" and "msgstr"). Then converted it to a .mo file and added the file in
/usr/share/locale/(your own language folder; in my case it was bn as in Bengali)/LC_MESSAGES/
The file name should be the same as the default greeter. For example if your default greeter is "pi-greeter" the file name should be "pi-greeter.po" the converted file name should be "pi-greeter.mo"