language selection and closed captioning

I don't think you could do this in the same lesson without a lot of fancy AS3 widgetry underneath the hood.  It would not be easy.

Your quickest and easiest way to implement such an idea is simply to have two versions of your course, one with English and the other in German,  Then you have a landing page that offers the learner the option (clickbox or button) to select their preferred language.  Clicking one or the other will branch them to open the relevant lesson.

Where you may encounter problems with this is when you are using an LMS.  If you have both English and German lessons packaged together in the same course, the LMS will think the learner has only completed half of the total lessons.  In that case, you are forced to have two entirely different courses packaged as an English SCORM and a German SCORM.  The user would then be selecting their preferred language in the LMS, not in the Captivate lesson.