Allow overriding translations from plugins

It would be great if plugin devs could use plugins to override translations of other plugins and the Moodle core.

Install a plugin -> Moodle uses the customized string
Remove a plugin -> Moodle immediately uses the original string

Example use case:
1. By default the login form says "Username"
2. Admin installs a plugin called "Login using phone number"
3. The form now says "Username / Phone"

This would make it much easier to install/remove plugins because the translations wouldn't have to be changed manually with the Language customization feature.