Let's start by re-writing your last sentence in English:
"A City or Capital in Europe, (After WW1), where you take the LAST 4 letters out of the word and set them aside; now the 2nd letter of those 4 letters, replace it with an O, and the result of the 4 letters with the new switched O creates a place or something you cannot lie (about? somewhere in which you cannot lie?)."
So, to look at it systematically--the nation of Yugoslavia was created--but Belgrade was it's capital, and that had been the capital of Serbia before the war, and it doesn't work for this riddle anyway. The nation of Czechoslovakia was created, with the capital at Prague, and that doesn't work. Hungary was separated from the dual monarchy, and its capital is Budapest, and that doesn't work. The Turks renamed Constantinople Istanbul, and that doesn't work. I don't know whether the mandates in the middle east count, but that would yield Beirut in the Lebanon, which doesn't work; Damascus in Syria, and that doesn't work; Jerusalem in the Palestine-Transjordan mandate, and that doesn't work; Baghdad in Iraq, and that doesn't work; Riyahd in Saudi Arabia, and that doesn't work.
The Russian empire had had its capital at St. Petersburg, which they renamed Petrograd during the war--but the capital of the new Soviet Union was Moscow, and that doesn't work. Poland, moribund since the third partition, was recreated, with its capital at Warsaw, unsurprisingly, and that doesn't work. Then there were the Baltic states--Estonia with a capital at Tallinn, and that doesn't work; Latvia with a capital at Riga, and that doesn't work; and Lithuania, with a capital at Vilnius, and that doesn't work.
So, i am frankly stumped. Have you quoted the riddle exactly as it was given to you? The remark about after World War I (it is usually written with a Roman numeral, not an Arabic number) implies a newly created city or capital of a newly created nation, and as far as i can think, i've named them all.