At this point there is no way to customize the course enrollment notification email without some custom coding to do so.
To edit the course enrollment email you can follow these steps:
1. In the site administration menu, click on Language
2. Then click on the Language Editing option
3. In the page that appears, click on the Edit Words or Phrases link
4. In the dropdown menu, choose Moodle.php
5. Press Control-F so your browser will search for the following text: welcometocoursetext
That will take you to the specific section where you can edit the course enrollment email.
No need to change language strings. In Moodle 2.3.1 you have ENROLMENT METHODS in every course. So you can change the custom WELCOME Message inside the SELFENROLMENT Method:
Theoretically it should be possible to deactivate the standard welcome message here by unchecking the option SEND COURSE WELCOME MESSAGE.
HtH, Rosario