For sure we will see airlines in Russia operating this. The question is which markets outside Russia will be penetrated by the MS-21. The SSJ100 did well in Mexico and Southeast Asia. Those two are likely candidates for the MS-21. I'm not putting off China yet since Russia and China will be collaborating on a widebody airliner which may share same technologies as the MS-21. Hence it is still possible for airlines in China would be ordering the MS-21 for commonality with the widebody. Europe is also a good market. LCCs and Charter airlines there could use a new large, short-range, narrowbody that is a cheaper alternative to the A320/A321. Alitalia had the SSJ100 on order but they had to cancel due to the recession. Latin America is also a very good market.
I wouldn't called it it did well on Mexico or Southeast asia. Many orders from those countries are being canceled because the Airlines went bankrupt or because it got canceled. Interjet order 30. Airlines in Southeast Asia are canceling and going bankrupt. Problem with Charter airlines and LCC is they are going bankrupt right and left especially in countries like Indonesia. So I wouldn't hold my breath if I was them.
As much as China like Russia (even though they don't really, they just want their money). They would make sure they sell their own products first.