Why are foreign language registrants redirected to home page after registration?
I wonder if your issue could be related to the language specific homepage (frontpage).

The relative URL to display as the front page is specified within admin/config/system/site-information. Make sure you specify a value for it that is different for each of the site's languages. To do so, you have to make it a "multilingual" variable (it should say "This is a multilingual variable.") in the description of that variable on that page.

For more details about this, refer to my answer at How to make content translation available for front page?