Russia

The Soviet Union did not collapse in the 20th century in the Fallout world and still existed in the 21st century, prior to Great War.

The Soviet Union had consular service and very likely foreign service, including consular relations with USA. Very likely Soviet Union wasn't at war against USA in 2077. However it is unknown whether during the Great War the Soviet Union was an ally of China, its enemy, or was neutral towards it, and it is unknown whether it still had its sphere of influence in Europe.

The name of the state still confirmed its continuity, so it still likely had a kind of quasi-federal communist-like system of government in the 21st century.

The Soviet Union is not mentioned at all in other parts of the background, especially in background concerning the Great War.

AK47, a Soviet assault rifle is a common weapon in Fallout Tactics.

Wreck of Soviet space station Mir appears as an easter egg in a special encounter.

Appearances
The only in-game mention of Soviet Union is in the biography of Natalia, one of the premade characters in Fallout, whose grandfather was a Soviet diplomat.

In Fallout 3 you can encounter Dukov, ex-mercenary Russian speaking with a heavy Russian accent.