Pasting from Wikipedia:

"Магистральный Самолёт 21 века" – "Magistralny Samolyot 21 veka"  translates as "mainline aircraft of the 21st century".

The correct transliteration is therefore MS-21. The big "MC-21" painted on its fuselage is in the Cyrillic alphabet.  According to the same source, in service it will be known as a Yakovlev 242 (Yak-242).