Dima,
I'd still go with Wikipedia:
http://en.wikipedia.org/wiki/Monomakh's_Cap
or Britannica Online Encyclopedia:
http://www.britannica.com/EBchecked/.../Monomakhs-Cap
Google:
http://www.google.ca/search?hl=en&source=hp&q=monomakh%27s+cap&btnG=Goo gle+Search&meta=&aq=f&oq=
Another possible (and legitimate) translation is 'The Crown of Monomakh'.
Given the controversy (as it often happens with 'foreign' variety names), it is always better to stick with the original name in the original language and provide 'possible' translations in the description of a variety. Otherwise, some folks in the future may think of 'Monomakh's Cap' and 'Monomakh's Hat' as two different varieties...
Tania