In German, the interjection »na« is used to express eagerness (»Na los, komm schon!«), surprise (»Na, so was!«), doubt (»Na, ich weiß nicht.«), denial and indignation (»Na, was erlauben Sie sich!«) and the emotion of the imagination of someones reaction (»Na, da wird sie aber Augen machen!«).
Another usage is as preamble of a phrase:
- hesitating agreement: »Na schön, von mir aus darfst du bis 9 bei Sabine bleiben.«
- conciliation: »Na, so schlimm ist das nun auch wieder nicht.«
- threat: »Na warte, Bürschchen, dir werd' ich's zeigen!«
- provocation: »Na und? Was willst du dagegen machen?«
- confirmation: »Na, und ob ich dort war! Frag Erich, der war auch dabei!«
- prompt: »Na, dann los!«
- renunciation: »Na, dann eben nicht.«
- intimate salutation: »Na, du?«
- question : »Na, was hast du auf dem Herzen?«
- answer: »Na, dass ich doch nicht bei Paul sondern bei Anna war.«
- evidence that a prediction became true: »Na, ich hab's ja gesagt!«
There is evidence, that this interjection was used since the 16th century, but its etymology is still unclear. Some say, it has developed from the adverb »nun« (Old High German, Middle High German and Old English: "nu", modern English: "now"), but even more experts believe it has developed independently from a sound that many people make in their speech in short breaks of disconcertedness.
»Na« can always be used singularly, but in some cases also doubled:
- Na, na, ich weiß nicht.
- Na, na, so schlimm ist das nun auch wieder nicht.
- Na, na, dann eben nicht.
But when ever it is doubled, it is used in some sense of denial. This is because in the doubled usage, it adds the meaning of »nein« (English "no"), which becomes »na« in colloquial speech in regions where Alemannic and Bavarian dialects are spoken (Switzerland, Austria, Baden-Württemberg, Bavaria). But in northern regions, »na, na« becomes »ne, ne«:
- Ne, ne, ich weiß nicht.
- Ne, ne, so schlimm ist das nun auch wieder nicht.
- Ne, ne, dann eben nicht.
But the meaning is not exactly the same. When you use the northern version »ne, ne« you only have the negating meaning, without the meaning of English »now, now«. When you use the southern variation »na, na« you have both meanings merged together: »now, now« and »no, no«.
So, south of the Speyer line people usually don't say »ne, ne«. They only say »na, na« and they mean both, »now, now« and »no, no«.
When people living north of this line say »na, na« they only mean »now, now«. If they want to express »no, no« they say »ne, ne«.
The German word »nun« and the English »now« are cognates. As already said before, both of them derive from the same root, which is »nu«. But, it is unclear if »na« is really related to these words.
Sorry, I don't know anything about the etymology of Russian words. You could ask your question about it in russian.stackexchange.com or maybe also in linguistics.stackexchange.com.