Hello Shivoham,
It was as you said, I forgot to check the inactive users (odoobot, defaultusertemplate, portalusertemplate and publicuser).
Also after changing those it asked me to change the contact preferences as well.

It didn't work for me. I have change the Language of all Users. including odoo boot and other inactive and internal users, And still it shows an error "Cannot deactivate a language that is currently used on a website."

There you have it, you need to change it from the website.