How to make a gmail app to scan words and trigger a popup reminder before sending an email?

I want to write a gmail extension that once the user clicks “send”, it scans the email for unclear language and provides a popup of suggestions to make the email more clear before verifying they want to send it as is.

A similar feature is when gmail provides a popup suggestion reminding you to include an attachment after a user hits “send” because it sees that there is no attachment, even though it was referred to in the email body text.

Can this be done as a gmail plugin (gadget) or rather as an extension for Chrome (and/or Firefox)? Which is the right API to use here? I am wanting to build a prototype for a school project and would appreciate any advice. Thank you!