You can change that placeholder text by changing <main folder>\deployment\web\js\login_i18n.js
Of course, when redeploying after a change, this JS file will be overwritten. So, to make sure your change in login_i18n.js sticks, add in your theme folder an extra folder called js and in that folder add the altered login_i18n.js file. When redeploying mendix will pick your changed file.
I've just tried the solution Ivo provided (copied the js folder from my deployment to the theme folder). When I made some small changes to both the login.js and login_i18n.js. When running the app, I noticed that login.js was updated in my deployment folder but login_i18n.js was not. Renaming the file (in theme folder) to custom_login.js did the trick. It created a new file in my deployment folder, and the changes were shown properly on the login page.
I'm using modeler 7.16.0