Who you also consider "non-citizens"[1], but not in the sense that they're citizens of some other state, you just refuse part of country's population of some citizen rights (like voting) as if it was Apartheid all over again.
It's a long-lasting grievance that could perhaps be eased by some diplomacy, but is instead being used by all aggrieved parties to further antagonist, and nationalistically protectionist agendas. There is little practical incentive for either Latvia or Russia to soften first, for reasons more than just national pride: this is a power play like any other. In truth, the status, while somewhat derogatory, is not particularly horrific, and impacts people's daily prosperity or international mobility little.
It's disingenuous to try to frame it as a human rights violation; it's most certainly not, but it is a festering issue that doesn't make reconciliation any more likely or easier.
> There is little practical incentive for either Latvia or Russia to soften first
Let me try to understand. Latvia calls some of its citizens "non-citizens" and somehow Russia (a different country) has the option of "softening first". By doing what exactly?
What are the issues that Russia should go soft on in order for Latvia to stop considering a fraction of its population "non-citizens"? And why it was an issue in the 90's where the Russia was famous "The Coma State"?