This is also posted to Translatable module (silverstripe/silverstripe-translatable#90) , but I assume this is more a porblem of userforms . The problem :
Having installed user-forms and translatable got an error when trying to "Delete all submissions" or "Delete Submission" in "Form Submissions" tab.
The error says : "Error at line 312 of [DOMAIN]/translatable/code/model/Translatable.php"
Problem is in static function set_current_locale($locale) function. Debuging $locale variable would show :
Debug (Translatable::set_current_locale() in Translatable.php:310)
ERROR [User Error]: Uncaught InvalidArgumentException: Invalid locale "en_US/deletesubmission/?id=2"
IN POST /admin/pages/edit/EditForm/field/Reports?locale=en_US/deletesubmission/?id=2
SO it takes locale as 'en_US/deletesubmission/?id=2' .
I'm not sure if this is your bug , or is this translatable module bug, so please redirect or answer me if possible
Submissions are now managed through the core GridField so all responsibility for dealing with that should be dealt with outside the module but the GridField IMHO