In the questions SLOVISK quotes, the sense is "could you tell me please ...?", "Would you go there with me?" and "Would you do it for us?". The same questions could be asked in the same meaning phrasing the verb in the affirmative (so without the "ne"). The effect of phrasing the verb in the negative is to convey the speaker's deference and make the request more polite.
In English, we can't reflect this deference or politeness by phrasing the question in this same wording in the negative "Can't you tell me please?", Wouldn't you go there with me?" or "Wouldn't you do it for us?". We'd have to say "Would you be kind enough to ...?" or "Do you think you could ...?" or something similar.
Interestingly, we could, however, make the request deferential by using a negative verb with an appropriate question tag in this kind of wording:
- You couldn't tell me ... (e.g. what that sign says), could you?
- You wouldn't go there with me, would you?
- You wouldn't do it for us, would you?
As I understand it, that's what SLOVISK is asking about, and actually it's a common feature in many (European) languages, including the Slavic ones that I have knowledge of, so I don't think it's a syntactic borrowing from German, as such.