It started off only using the USA's National Weather Service as a source[0] but gradually added international support[1]. But even then, outside of the US/UK, you would have been better off finding an app that you know uses your region's weather stations.

[0] https://www.kickstarter.com/projects/oliviadam/dark-sky-hype...

[1] https://web.archive.org/web/20190110174010/https://darksky.n...