We are a bit offtopic, but you may know that you cannot write down Latgalian identity in the Latvian national passport. Latgalians are prohibited to officially exist in Latvia 2.0.
And you very well can fix officially your Latgalian identity in Russia, surprise! Isn't it strange?
I don't find it strange at all, it plays very well into the "divide and conquer" approach to ethnic politics that Russia has put to use both in the past and more recently in Georgia, Ukraine, Moldova and elsewhere. For what it's worth, I disagree with there being ethnic markings in the Latvian Passport to begin with. I am in complete favor of removing them, not least because it would prevent Russian state from winning such cheap propaganda points.
And you very well can fix officially your Latgalian identity in Russia, surprise! Isn't it strange?