Well, we didn't get it from the Vikings, of course: this word is of direct lineage in Scots use, to the old English ‘cennan’, seemingly prior still to proto-Germanic—very much predating any kind of Viking influence. :)
That said, I think you're reading more into my statement than is really there :P I said I realised where it came from, not that I realised it came from German. Of course it came from a shared root with the German word. This usage just made it obvious to me, where beforehand I hadn't thought about it being so old, given it can be somewhat proscribed here in the wrong circles. :)
In some informal settings (certain university environments, relaxed cafes and bars, etc) it is common to use "du" with strangers. Getting a feeling for those conventions may simply take some real world practice out in the field :) Still you're right, when in doubt, it's probably better to use the formal "Sie". Thanks for asking!
English has a psychotic love affair with auxiliary verbs (words like "do" and "have"), but German does not. When asking a question in German, you only have to put the conjugated verb first in the sentence, no other words are required. Thus "Kennst du uns?" translates to "Do you know us?".
I think the answer "You know us?" is actually a correct translation as well, but Duolingo doesn't like it because that phrasing is used more to confirm information you already have than to actually ask a question.
'Do' comes from gaelic. As successive conquests of Britain brought in more languages, the commoners didn't just change language over night, words were slowly added to the common tounge when they were useful. This is why so much of English resembles Roman, germanic and French languages, but our awkward grammar is unique
No, the verb goes in the front in a simple yes/no question. More here: http://en.wikipedia.org/wiki/German_sentence_structure
You are hearing it slightly wrong. In German, a lone 's' sounds like an American English 'z'. The German 'z' has a 'ts' sound. So 'salzig' is pronounced closer to saltsig. You might be able to hear this better in a word like Weizen. Also try pronouncing the word 'ten' but add an 's' after 'T', so you get 'tsen' but say it very tersely.