plati.ru and similiar markets for other regions?

I should have been more specific: their promotional prices are generally very cheap and better than Steam. Argentina is a special case because they are in a severe economic recession.

I can also certify that nuuvem prices are generally much cheaper than what you would get on steam. Basically, the prices in nuuvem are the prices in Brazil, (so the prices in Brazil Steam vs Nuuvem are the same); Brazil's prices are generally 40-60% cheaper than the base price in USD, which means that, with the exception of Argentina (due to their devaluation and steam never adjusting prices), Brasil (because the price is the same) and Mexico (because prices are generally very similar), nuuvem has cheaper prices than in the rest of South America.
There are still some countries in latam where steam hasn't implemented regional pricing, paying full prices in US$ (I think these are all the countries in the steam latam region that don't have regional prices yet: Bahamas, Belice, Bolivia, Ecuador, El Salvador, Guatemala, Guyana, Honduras, Nicaragua, Panamá, Paraguay, Surinam and Venezuela) so the deal with nuuvem is even better.
Nuuvem also has sales throughout the year like any other store, and usually they also throw additional 10% coupons during some of them (the last ones I remember were during steam summer sales, and Carnaval).
The downside is that they lack many games.

A few Polish retail keys exist with locks, but it's rare.

Haven't encountered any Poland restricted key yet.