Sadly not possible since V16. User created translations were removed in favour of translations requiring the PO file to be embedded in a custom module.

I couldn't achieve it with the new v16 translations / po framework, as Chris Mann says. This is a real business risk Odoo, it's too easy for users to send messages externally.

I am also in need of this in a bad way. Our users are not only accidentally sending messages to customers instead of logging notes, but they also click Send Message, then write a message that should go to a customer, and instead of clicking the Send button below, they click Send Message again.

The problem is that the compose box in the chatter folds away when you click Send Message, so users think the message went out, but really it did not.

I want to change the Send Message button to Compose Message.

Cliff, please raise this as a support ticket and/or talk to your sales rep to get awareness. Odoo need to look at this, as Ray's solution doesn't work in v16. It causes a real business risk.