I made some researches about this time and can say that Stalin was a unique person. People hardly know his excellent abilities to remember and to system the information. Alas, people know much more about GULAG and all that was connected with repressions in a Stalins time.
He was born in a small town in Georgia and did not know any other language but Georgian till he entered seminary. There he learned Russian. He also learned and knew pretty well English. I just cannot say for sure where exactly he learned it. But while he was in prison (before the Great October Socialist Revolution) he asked his comrades to bring him books in English “not to forget the language in walls of prison”. This fact was also confirmed by Roosevelts son. Elliot Roosevelt remembered that Stalin knew English but preferred not to show it. When any question was asked during conferences Stalin always waited till the text was interpreted and it gave him extra seconds to think over the answer.
After Stalins death his personal library (around 20000 books ) was given to a state library in Moscow. Judging by the marks made in books Stalin definitely worked over 5500 books learning and researching different topics. It made him a specialist in different subjects. He was also able to quote pieces of literature by heart.
Now I see that I have pretty much material for a full -scale hub devoted to him. Hope to write it soon. :)