This might be answering your question too literally, but...........

The US State Department has actually cataloged communications received by foreign nations in response to the assassination of Abraham Lincoln! So you can read the very words written at the time of the news of the assassination here: Papers Relating to Foreign Affairs, Accompanying the Annual Message of the President to the Second Session Thirty-eighth Congress, Part IV, Appendix to diplomatic correspondence of 1865; the assassination of Abraham Lincoln, late president of the United States of America, and the attempted assassination of William H. Seward, Secretary of State, and Frederick W. Seward, Assistant Secretary, on the evening of the 14th of April, 1865; expressions of condolence and sympathy inspired by these events.

Granted, these are diplomatic communiques so (I imagine) there is some ceremony and boilerplate stuff here. It doesn't show what world leaders might have thought behind closed doors, but it does show what they publicly expressed to the United States at the time of Lincoln's assassination.

Here's a tiny snapshot:

President Juarez of Mexico

We also received the day before yesterday the news of the total defeat of the confederate army on the 9th of April. The great pleasure this news afforded us was marrred by the very sad impression which the shocking intelligence of President Lincoln’s assassination produced upon us. That great misfortune has profoundly impressed me, as Mr. Lincoln, who worked with so much-earnestness and abnegation for the cause of nationality and freedom, was worthy of a better fate than the poniard of a coward assassin.

Prince Kung of China

I had the honor yesterday to receive your excellency’s communication informing me that the President of the United States had been removed by death, an announcement that inexpressibly shocked and startled me. But, as you add that on the same day the Vice-President succeeded to the position without any disturbance, and the assassin had been arrested, so that the affairs of government were going on quietly as usual, I hope that these considerations will alleviate your grief at the event, and you will be able to attend to public business.

Prince Gortchacow of Russia

The telegraph has brought ns the news of the double crime of which the President of the United States has fallen a victim and Mr. Seward barely escaped.

The blow which has struck Mr. Lincoln, at the very moment when he seemed about to harvest the fruits of his energy and perseverance, has been deeply felt in Russia.