The US worked with the UK on the project, and under the Quebec Agreement of 1943, the US not only needed to tell the UK their plans, but get their assent before using the bomb. You can consider the top-levels of the UK government essentially briefed on the issue.

No other Allies were given much information. Truman hinted at Potsdam to Stalin that a new weapon was going to be used, but didn't go into detail. Stalin already knew the details, in any case, because of his spies.

The Canadians, Chinese, and French were not, as far as I know, told anything about operational plans. Some Canadians knew about the Manhattan Project, because Canada did play a role in it, but as far I can tell they were compartmentalized outside of the use questions.

As for national responses, it depends on the nation and who one is asking the opinion of. The Chinese government, for example, was pretty happy — they had no warm feelings for the Japanese. Stalin was not particularly happy, because he feared the war would end before the USSR could get involved with it, and thus the USSR would lose out on key territorial gains. Stalin holed up in his dacha for a day, depressed. When it became clear that the Japanese were not going to immediately surrender after Hiroshima, he ordered the date of the Soviet declaration of war and invasion moved up.

The question of national "reactions" to the use of the bomb is generally hard to answer, because there are many possible nations and many different groups within them, and things like dedicated polls or surveys or even easy access to information is not always available to us today. In general the reactions of those who were against the Axis powers was positive but tinged with uncertainty and ambivalence about the future. The atomic bomb, if it was credited with ending the war, was seen as an essentially positive development, but what it heralded for the world going forward was understood as possibly being quite negative indeed.

Thank you for such a detailed response!