Links from here redirect me to my iTunes where I can buy gift cards but there show a note "Redeemable in German store only". The same story if searching for the answer on apple.com/ru/... .
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 & 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.