I assume that you are asking about the dispute between these two countries over the Senkaku Islands (called Diaoyu Islands by the Chinese), not about more general conflicts that exist between them. If so, there are two other countries that are more or less closely connected to the conflict.
In one sense, Taiwan is the country that is most closely involved in the conflict. Taiwan also claims these islands. Taiwan feels that the islands are part of China and that they are the descendants of the legitimate government of China at the time the islands were given to China by treaty.
However, the foreign country that really has the most at stake in this conflict is the United States. The US is the world’s only superpower and it has issues with the idea of China expanding its power out into the East China Sea. The US sees this as a threat to it and its allies in the region. Relatedly, the US has a treaty with Japan in which the US promises to help defend that country. The US has said that it takes no position on who the islands belong to, but says that they are covered by its treaty with Japan. This means that an escalation in this conflict could get the US involved because it is obligated to protect Japan.