@grishka @kytta It's a general rule of this ISO standard: the code comes from an endonym (the name of the country in a local language). Germany is DE and not GE, Spain is ES and not SP, etc.
(But there are exceptions since there are not many two-letter combinations available. Albania is not SK, since it was already taken by Slovakia.)
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.