Air traffic control by country
Entry rules are national. The right to work, the language used on the radio, the medical standard, and the employer all change with the country.
A useful country page names the provider, the year of the source, and whether the text describes a current campaign or an earlier one. A summary of one country is not a summary of the next.
These notes name the provider to start with. They do not list current entry requirements. Those belong on the employer's own pages, gathered under Employers.
United Kingdom
For civil air traffic control, start with NATS. NATS staffs many units. Some airport towers are staffed by other organisations, so check the unit you mean.
United States
Civil controllers are employed by the Federal Aviation Administration. Hiring is published by the FAA, campaign by campaign.
Canada
Civil air traffic services are provided by NAV CANADA, which also publishes controller recruitment.
Germany
DFS is the main air navigation service provider in Germany. Some airport units are staffed separately. Confirm the employer for the unit before you treat a German vacancy as a DFS vacancy.