A nation within a multi-national state can be self-determining in important ways, and to a sufficient extent that international law would not require an independent state / support secession.
The Bloc Québécois is of the view that Quebec cannot be self-determining, in ways that are sufficiently important to be determinative, within Canada.
Devolution in the UK -- "home rule" -- is an example of mechanisms for self-determination within such a state. The complex constitutional arrangements in Belgium, by which the major ethnic/cultural/linguistic groups have authority over their own educational systems and the like, are another.
Canada's constitution divides authority over various legislative areas between the provinces and the federal government. The provinces have education, civil law (which is why Quebec's civil law is still derived from European
droit civil), health care, and all sorts of other things, but not, for instance, foreign policy.
The International Covenant on Civil and Political Rights says this:
http://www.unhchr.ch/html/menu3/b/a_ccpr.htmArticle 1
1. All peoples have the right of self-determination. By virtue of that right they freely determine their political status and freely pursue their economic, social and cultural development.
2. All peoples may, for their own ends, freely dispose of their natural wealth and resources without prejudice to any obligations arising out of international economic co-operation, based upon the principle of mutual benefit, and international law. In no case may a people be deprived of its own means of subsistence.
3. The States Parties to the present Covenant, including those having responsibility for the administration of Non-Self-Governing and Trust Territories, shall promote the realization of the right of self-determination, and shall respect that right, in conformity with the provisions of the Charter of the United Nations.
The big questions, of course, remain unanswered: when is some self-determination not enough self-determination, and what happens when it isn't?
The Supreme Court of Canada produced an enormous decision on the question a few years ago:
http://scc.lexum.umontreal.ca/en/1998/1998rcs2-217/1998rcs2-217.htmlReference re Secession of Quebec, (1998) 2 S.C.R. 217
It's a fascinating read for anyone with a deep interest in the issues; the Court reviews the whole of human history as it relates to the question, more or less. ;)
The headnote summary:
2. Does international law give the National Assembly, legislature or government of Quebec the right to effect the secession of Quebec from Canada unilaterally? In this regard, is there a right to self‑determination under international law that would give the National Assembly, legislature or government of Quebec the right to effect the secession of Quebec from Canada unilaterally?
Question 2
The Court was also required to consider whether a right to unilateral secession exists under international law. Some supporting an affirmative answer did so on the basis of the recognized right to self-determination that belongs to all "peoples".
Although much of the Quebec population certainly shares many of the characteristics of a people, it is not necessary to decide the "people" issue because, whatever may be the correct determination of this issue in the context of Quebec, a right to secession only arises under the principle of self-determination of people at international law where "a people" is governed as part of a colonial empire; where "a people" is subject to alien subjugation, domination or exploitation; and possibly where "a people" is denied any meaningful exercise of its right to self-determination within the state of which it forms a part. In other circumstances, peoples are expected to achieve self-determination within the framework of their existing state.
A state whose government represents the whole of the people or peoples resident within its territory, on a basis of equality and without discrimination, and respects the principles of self‑determination in its internal arrangements, is entitled to maintain its territorial integrity under international law and to have that territorial integrity recognized by other states.
Quebec does not meet the threshold of a colonial people or an oppressed people, nor can it be suggested that Quebecers have been denied meaningful access to government to pursue their political, economic, cultural and social development. In the circumstances, the "National Assembly, the legislature or the government of Quebec" do not enjoy a right at international law to effect the secession of Quebec from Canada unilaterally.
Although there is no right, under the Constitution or at international law, to unilateral secession, the possibility of an unconstitutional declaration of secession leading to a de facto secession is not ruled out. The ultimate success of such a secession would be dependent on recognition by the international community, which is likely to consider the legality and legitimacy of secession having regard to, amongst other facts, the conduct of Quebec and Canada, in determining whether to grant or withhold recognition. Even if granted, such recognition would not, however, provide any retroactive justification for the act of secession, either under the Constitution of Canada or at international law.
Hope that helped. ;)