i've imported the brazilian portuguese strings of questionnaire module (http://moodle.org/plugins/view.php?plugin=mod_questionnaire) via the file import tool and commited to the repository, but the module does not appear in the translator list of components.
Sorry, I do not understand what you mean by "... but the module does not appear in the translator list of components". Please explain or provide a screenshot showing the problem.
In AMOS I can see that the pt_br questionnaire strings have 53 missing strings (out of a total of 300). Apart from that, everything works fine.
What do you think about prepend "mod_" in contrib modules?
It would be perfect. However, it turns out it's not as easy as it might look. This issue has its root in deep dark history of Moodle development where all activity modules had their string files (like assignment.php, quiz.php or workshop.php) in the same location with language files of what we call "core subsystems" now (like admin.php or grade.php). On contrary to all other plugin types, activity modules have never used the mod_ prefix for their string files (or functions, methods etc). Which leads to obvious issue - how should the AMOS know that "foobar.php" is a string file for activity module called "foobar" and not a core subsystem "foobar"?
This is a known problem and it actually already led to several conflicts. The most recent one, IIRC, is that introducing the new core subsystem "media" caused a collision with a contributed plugin with the same name.
For this reason, AMOS has to rely on the file name only when it displays the list of contributed plugins. We might, in theory, try to check the list of all core subsystems that has ever existed in Moodle and prepend the mod_ prefix for all others. But there is a risk of false positives (something is marked as an activity module while it is not).
Thanks for the detailed explanation, David!