Hmm, need more information on your configuration. I simply receive a pop-up stating: Subscription Plan : This field is required!.

That is what I would like to have instead !.
I have:
Registration settings under Settings -> Global
Show subscription plans at registration: Yes
Allow Free Registered Users (without subscriptions): No
Create Subscriptions also for free lifetime plans: Yes
Plan -> Workflows -> Registration CB workflows
Allow registration: Yes
Propose spontaneously plan at registration: Yes
User Email Confirmation: Yes, before payment
User Approval by Moderator: Default (CB)
Plan -> Access
Published: Yes
Allow new subscriptions: Yes
Allow access from site frontend: Yes
Visible for Group (and above this group): Public Front-end
I'm on Joomla 1.5.11 with CB 1.2.1 and CBSubs 1.0.1
I would say that we have two problems here, the one with the language and the fact that CBSubs is indicating that a plan is necessary but none is available when actually this isn't true since the user just tried to register without selecting a plan.