Russian Царь всея Руси doesn’t actually use plural, it’s simply “Tsar of all Rus.”

Do other European languages use plural in translations of this title, or is it an English-only thing?

Well there were Belarussia, Russia, and in a sense, the Ukrainians were the original Rus (from the Norse word for Rowers), a Viking kingdom in the area of Kiev which the Vikings had called Gardarige. http://vikinghistorytales.blogspot.ca/2013/11/862-roric-returned.html https://tidsskrift.dk/index.php/historisktidsskrift/article/view/34154/66036

I cannot say specifically as to Ivan's title, but traditionally the Eastern Slavic Language-speaking area was all considered "Russia." This area was further subdivided into multiple "Russias," namely Great Russia, which roughly corresponds to modern Russia; Little Russia, which very loosely refers to Ukraine; and White Russia, which refers to Belarus. Belarus's name translates to White Rus', reflecting this split.