Enable translation domain in registratin handler

Fix for #20.

I'm not sure if this is a bug in the core, but if a hook or event handler need translation domain, enabling can be done by loading the module.

Now all user registration process connected to Legal plugin (form and messages) is translatable.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20
| License       | MIT
| Doc PR        | no