Stéphane, Russia is RU because I suppose RO (from Rossiya) was already assigned to Romania when USSR fell apart. Which is also SU (clearly just abbreviation of the English name Soviet Union) instead of something like SS (from Sovetskiy Soyuz), this I'm not sure why.

And another useless bit of trivia: the St Petersburg airport still uses 3-letter code LED from Leningrad because apparently changing it would've been too much trouble.