This is a helper module to address some problems in Drupals language behaviour.
What it is about
The idea for this module comes from issue #163165. I just don't like how Drupal behaves and so I made a module to address that issue. But there were some more ideas / issues it might solve and I did extend it a bit further so we can make sure Drupal uses the language for emails and content we think is more fitting for the situation, The name "Registration Language" is a bit misleading now but as I can't delete modules from cvs that easily and just have to give it a name I will stick to it. "Language behavior settings" maybe a more fitting name for the module.
In Drupal 6 a new user that did register through the sites process would have no language set. The sign up emails wouldn't have a language set so we can't translate them and the user would have to set his language on the first sign up as the site would use the default language which might not be the language of the user. Please take a look at my comment in this issue. The thread goes off topic in the middle but I think the issue is quiet clear. To address this in Drupal 6 we can use hook_user and set the language on the "insert" operation. This module will let you decide what language to use.
The second thing this module tries to fix is the usage of $language in the user.module for emails. The user.module uses the current language as default and not the preferred language of the user. This could lead to mess up. For example a German admin blocks an English user and the user will get a German notification mail. Again this module gives you the power to decide what language to use.
Use the mail edit module for translation
This module does not translate anything! It just sets the language so it could be translated! Drupal internal translation system will do this by default but there are caveats. By submitting the user settings form once they fixed to the default language. But you can use this module to translate them properly: Mail Edit
Ideas / Comments
If you have any ideas or problems with language behavior please post them into the issue queue!