The people in question are ethnic Russians who CHOOSE to not get a citizenship. They choose this, beccause people in Baltics without citizenship have the right to travel both within the EU and in Russia without a visa. Just to make it extra clear, let me draw you a table of a citizenship matrix in Estonia:
Estonian citizen - can travel within EU, needs visa for Russia
Russian citizen - needs visa for EU, can travel within Russia
No citizenship - can travel within EU, can travel within Russia
In the 1990s Estonian citizenship was given to any resident who asked for it, zero requirements. The people who remain stateless today rejected the proposal. Nowadays its more difficult to get Estonian citizenship, but still within reach for plenty of these people. They just don't want to do it, again, because they have friends and family in Russia - and Estonian citizens can't freely travel to Russia.
They can still vote for local elections and work. So there isn't really a strong incentive to care.
The best incentive for getting an actual citizenship is for those who want to travel beyond EU & Russia, because the global world doesn't give these stateless people special rights.
It should only be possible, if that person was born in Soviet Union (that is, before 1990), and doesn't speak the official language of the country he / she is living in.
Because they've chosen not to pursue citizenship. There's a simplified path that anyone in this situation can utilize. I don't see how giving them the choice and allowing them to live and work in the country indefinitely even if they choose no is in any way "un-democratic".
And if you're suggesting that 30 years is not enough time to learn the absolute basics of a language, that is just utterly ridiculous. Not nearly as ridiculous as comparing this to the slave labor that built Dubai though.
I think as an outsider you are completely missing the point: if Estonians ever wanted to give the Estonian citizenship to the residents who are unwilling to learn the official language of the country, they would have already done that.
Because by giving somebody a citizenship, you give them the right to vote. And who would the Russian speakers vote for, if they don't speak any Estonian? Pro-Russian parties and politicians.
These people were never born in Estonia, they were born in the USSR.
These people never spoke Estonian, and never tried to speak.
Nobody ever asked them to stay in Estonia and to pay taxes in Estonia.
They are always welcome to leave, if they don't want to fully integrate into the Estonian society by learning the official language of the country.
Fundamentally, the ethnic Estonians don't owe anything to the descendants of their occupiers.
Technically, Estonia is not the successor state of the USSR, Russia is. If a person was born in the USSR, and only speaks the official language of the successor state of the USSR, then that person should probably be a Russian citizen, not Estonian.
https://en.wikipedia.org/wiki/Czech%E2%80%93Slovak_languages
Not true for Lithuania. Educate yourself.