I wonder if any international law or agreement authorizes the Taiwanese people with the right to self-determination. Namely "Taiwan's independence or not should be decided by Taiwanese people, not by any other country".
I would like to see the original context/reference for statement like this, whether China agrees with it or not.
The example of Kosovo brought up in SJuan76 answer in the comments is helpful. At the time Kosovo was a province of Serbia (and internationally recognized as such by (almost?) everyone). Kosovo declared independence from Serbia. Serbia asked the International Court of Justice to rule whether this declaration was lawful or not.
They key quote (from here) is:
ICJ President Hisashi Owada said international law contains no "prohibition on declarations of independence."
This would also apply to Taiwan or to the Donbass region in Ukraine. There are no international laws that regulate who is or isn't allowed to declare independence.
The follow up question is whether such a declaration is internationally recognized or not. Every country decides on their own which other entities they do or do not recognize as independent countries. There are no international laws about that and in some cases different countries come to different conclusions about the same entity.
There is no "court of international law" in the first place. Politics is merely the art of the possible. Once this truth is grasped, the question naturally dissolves itself.
If that is still not clear, for reference, this is the territory that Taiwan AKA Republic of China AKA "Chinese Taipei" claims sovereignty over:
No.
It is a circular condition.
If you claim that Taiwan's independence to be the right of Taiwanese people, then you are already recognizing Taiwan as an independent country, and not part of another country.
In general, countries are recognized as independent, regions are not. Again a full circle. When an entity is recognized as a country depends only on the opinion of the recognizing country, and certainly there are lots of entities whose state cannot be agreed upon, with different recognition levels by different countries (e.g. Kosovo).
For example, for Germany Kosovo is an independent country, while at the same time it considers the breakaway republics of East Ukraine as regions of Ukraine and not countries. As such, any independence of those entities should be decided by Ukraine, and dealing with them as independent countries would be an attack on the territorial integrity of Ukraine.
The more that has been decided was that the International Court of Justice ruled that Kosovo declaring its independence was not a breach of international law.
The charter of the United Nations says:
Article 1
The Purposes of the United Nations are:
- To maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace;
- To develop friendly relations among nations based on respect for the principle of equal rights and self-determination of peoples, and to take other appropriate measures to strengthen universal peace;
- [...]
- [...]
And in Article 2, section 4:
All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.
(emphasis mine)
So if Taiwan is a "state", the UN charter says that no UN member may deprive it of its right to self-determination, and in particular may not use the threat or use of force to do so.
This begs the question of whether Taiwan is a "state". As the charter itself does not define this term, let's consult what the Encyclopædia Britannica says about states in international law:
The accepted criteria of statehood were laid down in the Montevideo Convention (1933), which provided that a state must possess a permanent population, a defined territory, a government, and the capacity to conduct international relations.
Taiwan meets all these criteria: The defined territory is the island of Taiwan, permanent home to 23 million people, a government with all the usual ingredients, which demonstrates its capability to conduct international relations by maintaining the 31st largest diplomatic network in the world with 110 offices.
Appendix: Recognition of statehood
Britannica writes:
Recognition is a process whereby certain facts are accepted and endowed with a certain legal status, such as statehood, sovereignty over newly acquired territory, or the international effects of the grant of nationality. The process of recognizing as a state a new entity that conforms with the criteria of statehood is a political one, each country deciding for itself whether to extend such acknowledgment. Normal sovereign and diplomatic immunities are generally extended only after a state’s executive authority has formally recognized another state (see diplomatic immunity). International recognition is important evidence that the factual criteria of statehood actually have been fulfilled. A large number of recognitions may buttress a claim to statehood even in circumstances where the conditions for statehood have been fulfilled imperfectly (e.g., Bosnia and Herzegovina in 1992). According to the “declaratory” theory of recognition, which is supported by international practice, the act of recognition signifies no more than the acceptance of an already-existing factual situation—i.e., conformity with the criteria of statehood. The “constitutive” theory, in contrast, contends that the act of recognition itself actually creates the state.
There's no settled international law on declaring independence, but there are cases for both sides:
On the one hand, the UN still considers the Falkland Islands and Gibraltar, among other places, to be "non-self-governing territories" and subject to "decolonization", even in cases where there was no pre-colonial population and the current population voted overwhelmingly to keep the current arrangement. This suggests that external parties may have a say in the independence of a region without regard for the opinions of the locals.
Also, in the aftermath of both WWI and WWII, the victorious factions (re-) established a number of (at least nominally) independent states in eastern Europe and Asia without paying too much attention to the opinions of the locals (though much was said about self-determination during the process), generally by handing power off to some favoured faction and helping them suppress rival factions until they'd established enough control to stand on their own.
On the other hand, most places that are currently "independent" are so because some faction of locals wanted to be independent in some way:
Well, the Han people invaded Taiwan en masse during their civil war in the mid 20th century, overwhelming the local indigenous Austronesian "Formosans" by numbers. Before that, it was administered by Japan for many decades, and only small parts of it were ever under the control of Qing dynasty proxies (e.g.: the pirate Coxinga), whilst indigenous small "tribal" states existed, even through preceding Dutch and Portuguese loose administrations. It's difficult to see what (or whose) law applies, or exactly who might be seeking independence from whom.
If for example, many countries refuse to recognise the independence of a large powerful country, then what meaning does it have? Similarly, if many countries acknowledge the independence (or right to self-determination) of small (in numbers or area) polities (or "countries"), what meaning does it have? Independence seems to be more a de facto state, that is latterly given de jure status, as it can't practicably be changed. Then there's the related point about economic independence. You could argue that most countries are not economically independent.
So, to answer the question... there is only any international law that says anything because powerful countries agree that there is, and in the case of Taiwan, at least one powerful country doesn't agree; but other powerful countries do agree; hence a war may ensue to decide it.
