How to buy a gift card from a German store for a Russian store?
Gift cards can only be redeemed in the same country/region that it was purchased. You can't redeem gift cards outside the country or region.

According to this Apple Support page:


  If you're trying to redeem an App Store &amp; iTunes Gift Card or Apple
  Music Gift Card that was purchased in a different country or region,
  make sure that you're redeeming it in the iTunes Store for the same
  country or region. You can't redeem cards outside of the country or
  region of purchase.