As to your statement about "No judgement on anyones part" that would seem to be a dog chasing it's tail. How could it be? Someone is making some kind of judgement just to be considered for the challenge. You are making judgements every second of every day as to what is real or not.
If an entity responds in German, then a "Judgement" must be made by someone that it really Was German. Explain that?

people may make as many judgements as they like but, when it comes to the determination of whether you passed or failed, no judgement should be necessary. This stipulation is part of the protocol design.
Asking someone, even an expert, to decide if a person is German based upon their speech alone requires judgement upon their part. There is room for subjective opinion and a second expert could conceivably disagree. What if the person was born in Austria but had lived in Berlin for 10 years etc?
A protocol requiring no judgement should be self evident when it comes to assigning success or failure. Take the dowser trials, for example, either they find the concealed material x times out of y or they do not. There is no room for debate or disagreement over their passing or failing.
This is why EVP claims are hard to test, say, for example a question is asked whose answer is bananas. If there is room for someone to listen to a recording of the answer and say, "hmmm, well it sounds like containers, or maybe manyanas," then they are having to make a judgement and that would mean that your protocol was flawed.
If you were to say, when 10 people listen to the answer, at least 8 of them will agree, without prompting or intercommunication, that the voice said bananas. In that case, if 8, 9 or 10 of the listeners wrote down the word bananas and only bananas then it would be evident, without judgement that you had passed.
The above is all moot, though, until you describe, in detail, exactly how you will ensure that the voice appearing on the tape was not a trick.
How, precisely, will you rule out all forms of trickery?
Even with a writer working for you, the above question is one you will have to answer for yourself. Why have you been avoiding it thus far?