Although it is possible to have Choice fields which are optional (required=False) the default message "(no value)" in the rendered form suggests that the user is required to select a value. This is particularly apparent in the German translation which is a literal translation of the message id and not the default: "vocabulary-missing-single-value-for-edit" as opposed to "(no value)". This is confusing for users who know nothing about Zope vocabularies or sources.
I suggest that the default be "(nothing selected)" which is a better description especially if a user wishes to have no value selected for a field. The enclosed patch has a new default "(nothing selected)" for both single and multiple select fields and the relevant German translation.
Changed in Revision 111361, 111363 and 111364