The default rules are good: you can't create a translation package for a particular language when that language is already in the status "In Translation". And in the Translation Dashboard, the context menu action to create a package is disabled in that case. So far, so good.
In the create translation package dialog box, however, all languages are available for selection. The translation status is ignored. It would be better if it was applied in this window by hiding the check box for those languages (not the language itself, just the checkbox). This would give correct input to the user and avoid unnecessary package creation failures. Adding a refresh button in this window would probably be a good idea for performance reasons.
Even better would be to have the "create/cancel translation package" actions from the context menu available as actions at the top of the Translation Dashboard so that a user can create several translation packages at once from a view where he has all relevant information. The behavior should be the same as the context menu actions: if a language is selected for which the action is not allowed, the action should be grayed out.