Maybe an investigation of video- or tape-recording, evaluated by Profil-Analyse (Grieshaber)  would be useful. Elicitating activities could be a retelling of an image or video-presented story. (cf. as source someting like in HAVAS-5, which also let you evaluate on vocabulary and other aspects).

You might want to assess (pre and post) student's communicative abilities (general or more genre specific). See for example a recent article by Mercer et al.: http://www.sciencedirect.com/science/article/pii/S0959475216301803 This might give you some evidence whether the program meets it goals (depending on what these goals are). 

you may want to asses using standard evaluation tools for programmes. There should be some available.I would also go with the ideas shared by the colleague from Leipzig

Evaluation studies can be tricky. I would ask firstly who teaches the learners German? How do they teach them? How do the learners receive them? how do they evaluate them? how do learners receive them?

Secondly, between the learners how do they communicate? is there power relation i.e. if some learners would regard themselves as teachers in the conversation/interaction or not? how do they communicate e.g. sentence construction, sign language, etc. How easy or not for learners to grasp the language e.g. is it based on how close their mother tongue is to German, or speaking it at home or not?

Nexus of these relationships: teacher-leaner, leaner-learner, learner-teacher informed by the classroom and home contexts.