What impact did WWI and WWII have on international diplomacy? What kind of long-term problems were created?

Your question refers to the two World Wars of the twentieth century. These wars were fought mostly in Europe but also across other regions like the Far East, North America, Africa, and South East Asia.
World War I occurred from 1914 to 1918. This war was fought mostly between opposing alliances. One alliance pitted the Austro-Hungarian Empire, Germany and the Ottoman Empire. The other alliance consisted of French, Russian, and British Empires.
World War II occurred between 1939 and 1945. On one side were the Allied Forces that included the British, French, and Russian troops while the other side has Italy, Japan, and Nazi Germany.
The Need for Peaceful Co-Existence
Both World War I and World War II caused warring factions to realize the need for peaceful existence between nations. The two world wars also led to the creation of organizations that helped to maintain peace through diplomacy.
The organizations that were created to maintain peace are the League of Nations after World War I, and the United Nations after World War II. The League of Nations, created after World War I, was an international diplomatic organization meant to help resolve disputes between nations.
Long-Term Problems caused by World War I
The Ottoman Empire was defeated and consequently partitioned by Britain and France. This left a major void in the Middle East resulting in political instability. It also saw the rise of dictators like Saddam Hussein and Muammar Kaddafi.
A lot of monarchs in Europe were overthrown or removed and replaced by new, unstable Republicans. These new republicans were weak and were eventually replaced. Germany saw the coming of the Nazi government, the Tsars were overthrown in Russia, and Fascism was on the rise in countries like Italy and Japan.
It is believed that the Ottoman Empire caused the Armenian genocide. Chemical weapons were used during the war and these have now been outlawed. War Crimes did occur resulting in the death of millions and wiping out of populations and cities.
Long-Term Problems Caused by World War II
The war left deep psychological and social scars across most of Europe. It also had a deep impact on the mental and physical health as well as quality of life for all those who experienced it. European economies were shattered and countries were left weak and ineffective. Balance of power shifted from Europe to the United States.
Atomic bombs were dropped in Hiroshima and Nagasaki, in Japan, resulting in hundreds of thousands dead, cities devastated, and numerous other challenges that linger to this day. Genocide committed by the Nazis against Jews in Europe caused upheaval in Europe. Minorities were targeted, marginalized, and moved by governments in the Soviet Union, United States, and Canada.
Further Reading https://en.wikipedia.org/wiki/World_war

Both World War I and World War II led to the development of international organizations intended to maintain the peace. The League of Nations, created after the First World War, and the United Nations, created after the Second, fit this description. But there were crucial differences related to diplomacy. One was that the League of Nations, established at the Paris Peace Conference, was weakened by the fact that the United States, having failed to ratify the Treaty of Versailles that created the League, did not join it. The League was generally powerless to deal with crises created by aggressor nations during the interwar period. Another issue was that the Treaty of Versailles took a punitive approach toward Germany, forcing that nation to accept responsibility for beginning the war. As a result, Germany was stripped of the bulk of its armed forces, lost valuable territory, and was saddled with high reparation payments that contributed to economic crisis in that nation. This, along with the national sense of humiliation in Germany, led to the rise of Nazism in the early 1930s. Nazi aggression, along with that of Fascist Italy and Japan, was free to flourish in the interwar diplomatic environment.
After World War II, the United Nations was implemented at the behest of the Allied Powers. It held a great deal more diplomatic authority than the League of Nations, in no small part due to the presence of the United States. But the United Nations in many ways became another forum in which a new diplomatic struggle—the Cold War between the United States and the Soviet Union—emerged. This new conflict became central to the global diplomatic order. The key to diplomacy during the postwar world was the development of atomic/nuclear power, which on the one hand made the many crises that broke out between the United States and the USSR highly tense, and on the other made global war so potentially destructive that neither side was willing to bear the costs. Another major diplomatic development in the aftermath of World War II was the advent of alliance systems. In Europe, the North Atlantic Treaty Organization (NATO) developed in opposition to the Soviet-led Warsaw Pact. In Asia, the Communist revolution in China led to a diplomatic restructuring of that region that saw the United States embrace South Korea and Japan as crucial allies.
https://www.theatlantic.com/photo/2011/10/world-war-ii-after-the-war/100180/