From the original issue
"If you view a user profile from your course and send him a private message the course language would be another than the language of the course.
Steps
Create a course and choose english for Language.
Choose Portuguese (or another) for the default language of the site.
List the participants of the course.
Choose one, click to see his profile.
Send him a message from the Send message link in user/view.php
The message form would have the Portuguese language (it was expected to be English language).
Why this is bad ?
If you invite an English teacher to run a moodle course in Brazil for example, he will see some screens in Portuguese."