I am experiencing some trouble here with SilverStripe 2.4.7 and Translatable when pressing Return in single-line CMS fields. Sometimes the current record is being saved, sometimes a new translation is being created and sometimes I get a PHP exception that requestVar() was called on a non-object in SecurityToken.php probably because of "if(!SecurityToken::inst()->checkRequest($request)) return $this->httpError(400);" in CMSMain's createtranslation(). The funny thing is that in Google Chrome for example, pressing Return does absolutely nothing while in Firefox I get the described behavior.

How does your SilverStripe behave? Also try creating a translation and then re-testing.