I use events+ with WPML for multilanguage websites.
I discovered that sometimes trailing slashes are missing.
I thought it was an issue with WPML.
I've open a support ticket on WPML website.
Here is their answer:
In the end the problem looks like a "plugin / WPML incompatibility" issue.
I would really recommend that you contact the plugin author and ask him to join our "Go-Global" program as mentioned here: http://wpml.org/forums/topic/sometimes-trailing-slashes-are-missing-in-default-language/#post-95118
Can you arouse interest in the author by letting him know about the program?
This will benefit both you and other users of the same plugin.
Could you verify this with the developer ?