Are there any international standards or declarations/conventions on prisoners' right to vote?
The International Convention on Civil and Political Rights (ICCPR),1 Article 25, says:
Every citizen shall have the right and the opportunity, without any of the distinctions mentioned in article 2 and without unreasonable restrictions:
(a) To take part in the conduct of public affairs, directly or through freely chosen representatives;
(b) To vote and to be elected at genuine periodic elections which shall be by universal and equal suffrage and shall be held by secret ballot, guaranteeing the free expression of the will of the electors;
(c) To have access, on general terms of equality, to public service in his country.
The United Nations Human Rights Committee (the monitoring body for the ICCPR) has said:
The right to vote and be elected at genuine periodic elections must be established by law and may be subject only to reasonable restrictions, such as setting a minimum age limit for the right to vote. States parties must take effective measures to ensure that all persons entitled to vote are able to exercise that right, including measures to overcome specific difficulties, such as illiteracy, disability, language barriers, poverty and impediments to freedom of movement. The Committee has taken the position that a blanket ban on prisoner voting is incompatible with article 25.
The Supreme Court of Canada has summarized (in a dissent in Sauvé v. Canada, 2002 SCC 68, but the summary is accurate):
Article 25 of the International Covenant on Civil and Political Rights (“ICCPR”) states that every citizen shall have the “right and the opportunity” to vote “without unreasonable restrictions”: ICCPR, 999 U.N.T.S. 171, entered into force March 23, 1976. The United Nations Human Rights Committee, in a comment on Art. 25 of the ICCPR, stated that restrictions on the right to vote should be “objective and reasonable” and that “[i]f conviction for an offence is a basis for suspending the right to vote, the period of such suspension should be proportionate to the offence and the sentence”: “General Comment Adopted by the Human Rights Committee under Article 40, Paragraph 4 of the International Covenant on Civil and Political Rights”, General Comment No. 25 (57), Annex V, CCPR/C/21, Rev. 1, Add. 7, August 27, 1996.
The UN Human Rights Committee, in its Observation Report on the United States in 2014, reported that the United States was not meeting its obligations under this Article:
...the Committee reiterates its concern about the persistence of state-level felon disenfranchisement laws...
The State party should ensure that all states reinstate voting rights to felons who have fully served their sentences ; provide inmates with information about their voting restoration options ; remove or streamline lengthy and cumbersome voting restoration procedures ; as well as review automatic denial of the vote to any imprisoned felon, regardless of the nature of the offence.
1. 173 nations are parties to the ICCPR. It is a treaty and as such is binding upon the parties to it and must be performed by them in good faith.
International law doesn't really exist.
Some people talk about international law, but let's be honest: international law doesn't really exist. Every state has the sovereign right to rule its territory as it wishes, and if another state wishes to change that, the only real options are coercion through economic and military might. Ultimately, the only rule of international law is the law of the jungle: the strong do as they wish, while the weak either kneel or are forced to their knees.