(Since you specified them in the description). I'm not a native speaker, but I've been studying slavic languages for about 4-5 years now. If anyone wants to add to this feel free. I did my best to write these cyrillic names out using the roman alphabet.

I'll offer a few suggestions for each, and here's a brief explanation why.

A lot of medieval Russian principalities simply used the capital city to refer to the entire state, giving us the english names: Grand Duchy of Moscow, Republic of Novgorod, etc. But in Russian, they use the adjective form and drop the 'of' - giving us something like Moskovite Grand Duchy in english. Using the Russian/Old Church Slavonic makes it more complicated, as the suffix of the Adjective changes depending on the 'gender' of the word. So we get Moskovskaya for feminine nouns like republic, but since the Russian word for principality is neuter, we use Moskovskoye when referencing it as such. Furthermore, it was popular for rulers to just add the word "Grand" to their official title in an effort to out-class the neighboring princes.

Western and southern slavs have their own rules too, but it looks like you got those mostly right. I may look deeper into the smaller countries in those regions if need be.

Format Guide:
- Standard EU4 Name -
Suggested Names (Explanation)
extra notes

- Russia -
Rossiya (Modern Russian for Russia)
Rossiskaya Tsarstvo (Russian Tsardom, would cease to make sense on governement form change)

- Ruthenia -
Rus' (It is meant to represent a resurgent Kievan Rus', so I took the Old Slavonic name for the same state)
Rus'kaya Zemlya (Alternative OCS name meaning 'Land of the Rus')
Rutheniya (Latinised)

- Novgorod -
Veliky Novgorod ("Great Novgorod" in modern Russian, more generic name used for all periods, not just eu4's timeline)
Novgorodskaya Zemlya ("Novgorodian Land" using OCS for contemporary name in 1400s/1500s).

Notes: You have it as Novgorodskaya which I believe is the feminine adjective meaning 'Novgorodian'. If you add a feminine noun (either Zem'lya/Zemlya for land or Respublika for republic) it'll be correct. Novgorod by itself is the Russian noun for the city as well.

- Muscovy -
Moskva (Simply "Moscow" in modern Russian/OCS)
(Velkoye) Knyazhestvo Moskovskoye (Muscovite (Grand) Principality)

Notes: This one is super tricky because the name changes so much in the time period of EU4 (they basically kept adding to the title of the Knyaz every time they conquered another prince). And people living there didn't necessarily have a uniform national identity yet. I'd personally just use Moskva as that relates most closely to the Identity of the Tag in EU4 (referring to the grand prince from moskva) without being excessively long. Moscoviya/Moscovia is the westernised version, based on the latin word for Moscow back then.

- Perm -
Velikaya Perm' (Russian: Great Perm)
Uedzhit' (????) Perem (Komi-permyak for Great Perm. I don't know if I romanticised the 'great' part of that correctly)
Perem (Just the 'Perm' part from the Komi)

- Zaporozhie -
Viysko Zaporoz'kye (Ukranian + Cossack Dialect: Zaporozhian Army/Host)
Zaporozhtsi (Ukranian: [The] Zaporozhians)

Another hard tag to name. Since EU4 made them a horde, I'm using Horde-like names

- Beloozero -
Beloozera/Belo Ozera (Means white (belo) lake (ozera) in Russian, the term for the geographical region this state is based in)
Beloozerskoye Knyazhestvo (Literal name for the Principality of White Lake, Russian)

- Polotsk -
Polatsk
Polatskaye Knyachva (Belarusian, hence the slight spelling changes here)

- Chernigov -
Chernihiv (Ukranian name for the city itself)
Chernigovskoye Knyazh'ctvo (Old Church Slavonic, from Kievan Rus' times. Chernigov Principality)

- Smolensk -
Smolensk (Russian City name)
Smolenskoye Knyazh'ctvo (Same as above, OCS)

Notes; these last two really only existed underneath the Kievan Rus' as semi-independent princes. That's why I went with the Old Church Slavonic names for the Political Bodies. Which option you choose just depends if you want to play them as restored princes from a lost age or a modern principality based in an influential, historic city. And it depends how long of a name you'd like, lol

- Kiev -
Kyiv (Ukranian City)
Kyivskye Knyazivstva (Principality of Kiev: Ukranian)

- Galicia-Volhynia -
Galitchka-Volynskye Knyazivstvo (Ukranian for the Principality of Galicia-Volhynia)
Galichina-Volhin' (Literally 'Galicia-Volhynia' in Ukranian)
Korolivstva-Rus' (Ukrainian alternative name meaning Kingdom of [the] Rus' - historical name leading up to being absorbed by Poland)

- Pskov -
Pskov (Russian City Name: No change here from the original)
Pskovskaya Respublika (Pskovian Republic)

- Rostov -
Rostov (Russian City)
Rostov Veliky (Great Rostov, popular nickname for the city)
Velikoye Knyazhestvo Rostovskoye (Rostovian Grand Principality)

- Ryazan -
Ryazan (same as above)
(Velikoye) Knyazhestvo Ryazanskoye (Ryazani (Grand) Principality)

Note: (you see where I'm going with these)

- Tver -
Tver
Tverskoye Knyazhestvo

- Yaroslavl -
Yaroslavl
Yaroslovskoye Knyazhestvo

- Odoyev -
Odoyev
Odoyevskoye Knyazhestvo

PS: Sorry if my formatting is off. It wont let me preview.

Man, thank you so much for this. You have no clue what of an amazing help you've been.

I'll push the update sometime tomorrow with your suggestions!

No problem my guy. You've got your work cut out for you so I figured I'd help out in an area of my (relative) expertise. I'll probably double czech (haha) the other slavs tomorrow or this weekend.

Sorry, I didn't see this thread until after I posted my comment, but (restating as I'm not sure my comment went through) as I said I don't think Byzantion is appropriate for Byzantium. The ERE never referred to themselves as anything other than 'Roman Empire' - the Greek for that, 'Vasileia Romaion' (Βασιλεία Ρωμαίον), is probably the most accurate endonym.

an addition to aaatos: Greek: Βασιλεία Ῥωμαίων, tr. Basileía Rhōmaíōn; Latin: Imperium Romanum
https://en.m.wikipedia.org/wiki/Byzantine_Empire

Can you change "Glogow" to "Głogów"? I'm a Polish speaker and thats how you say it.

Originally posted by aaatos:

Sorry, I didn't see this thread until after I posted my comment, but (restating as I'm not sure my comment went through) as I said I don't think Byzantion is appropriate for Byzantium. The ERE never referred to themselves as anything other than 'Roman Empire' - the Greek for that, 'Vasileia Romaion' (Βασιλεία Ρωμαίον), is probably the most accurate endonym.

I've already renamed Byzantium in one of the previous two updates

Originally posted by Filip32:

Can you change "Glogow" to "Głogów"? I'm a Polish speaker and thats how you say it.

Glogow has been renamed to it's Silesian name

Can you change Korea to Joseon or Chosŏn? That' was how it should be called during that time period.

Can you also change Ming to "Da Ming"? (And Qing to Da Qing, too), which means "Great Ming" in Chinese. Again, that was how the Ming Empire referred to itself historically. If Vietnam can be "Dai Viet" (Great Viet), why can't we make Ming great again?

There are many names for "The Palatinate" but "Die Pfalz" doesnt fit "Kurpfalz" or "Pfalzgrafschaft bei Rhein" would be much better the latter one might be a bit to long though.

Also adjectives would be really nice

Read the patchnotes

i dont get what you mean

Crete name isn't displayed properly (Kr?t?)

I'll be rolling some other suggestions here as it seems where most ppl are submitting theirs:

-Kazan: Qazan
-Nogai: Nogay
-Ethiopia: Ityop'p'ya
-Hercegovina: Svetog Save (as far as I know, Hercegovina is a modern name derived from the title "Herceg/Herzog" used by the rulers of the region, but at the time the state was called "Vojvodstvo Svetog Save", after the first archbishopric of the region).