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People in democratic countries get a vote in the governments which rule over them by rule of law. If you have a permanent address in a municipality, you typically get a vote for the municipal government. If you move to another city, you vote there. It makes sense to vote for the municipal governance to which one pays tax and which decides ordinances and regulations which applies to the resident. However, if one is not a citizen of that nation, one generally (with some exceptions) doesn't get a vote in the municipal matters, even if one pays the municipal tax to that municipality, and is living under the ordinances of that municipality. That doesn't make any sense reflected with the principle of no taxation without representation.
In national elections, whether parliamentary, presidential, referendums, or other democratic elections, one generally gets a vote based on country of citizenship. Thus people who pay taxes for the government in question, and who live under the laws of that government, don't get a vote if they are not citizens. Similarly, people who live outside of that country, but are citizens, are still eligible voters in those elections, even when they pay taxes to some other country and live under the laws of some other country.
Taking European Union as an example, although similar consideration apply to other regions of the world as well: In European Union people additionally get a vote in the EU parliamentary elections if they are citizens of an EU member country.
Regardless of where they live, even when living outside EU, they get a vote for the candidates of their own country of citizenship, to choose the EU parliamentarians representing their country of citizenship. Dual citizens should only choose one country to vote candidates in. People thus get a vote for a specific country and a union of countries, irrespective of whether they actually pay taxes for those governments, and whether they actually live under the laws decided by those governments.
Does it make sense that eligibility to vote is tied to citizenship in a world where people are globally mobile? Wouldn't it be much more just and proper, producing more effective democracy, to let those people vote who permanently reside under the governance of that government, and who pay taxes to that government to pay for its operation?
Citizenship tests typically require living in the country for a number of years, without criminal incidents, supporting oneself economically, and learning the local language. When people are globally mobile, this generates a number of problems. The most obsolete of these criteria is the language requirement. In the age of real-time language translation and international corporations, what sense does it make to require competence in a specific language?
National borders are set in stone by UN. Originally drawn based on human habitation, and defensible borders, roughly determined by ethnic and language differences of people, all these aspects have become increasingly obsolete.
Ethnicities freely mix in every place in the world except in apartheid countries. Languages are translated by machines, and used based on context, not based on location. Defensible borders are no longer such. Free movement of globally mobile people dilute the meaning of borders to the point where in some places they can only be a line drawn on ground if even that.
To create a more fair and just world, where people have democratic power over the governments they live under, people's eligibility to vote should be differentiated from their nationality and citizenship. People should have a vote for governments which they pay for in taxes, and which determine the laws people are subordinate to. To do otherwise is removing the proper democratic power of people over their own governance, and has no place in civilized world.
Real-time translation and multinational, globally interconnected business has made language requirements in citizenship processes obsolete. These should be removed.
Borders prevent free movement of people and cause people being prevented from self-actualization where they would best be able to do that. This causes artificial inequalities and creates unnecessary tensions between countries, causing conflicts and wars. We need to dilute borders by allowing free movement of people, which will be of social and economic net benefit for all, and also effects a great force for peace in the world.
We are all citizens of the world and we should understand and govern our pale blue dot through that lens.