How was YouTube able to deanonymize Tor?

Question

I was using the Tor browser (in safer mode in tails-5.2 with default settings, with no addon or any modification) to browse YouTube. While I was searching for Python tutorials, it offered that in the language of my region. How did YouTube figure it out even in Tor?

Note: I used a fresh tails with no persistent storage or any other modifications. Also, I never searched YouTube in my language or watched any content of my region before. Hence, YouTube should have no knowledge of my language, isn't it? I tried multiple times rebooting tails and searched (with & without bridge) again only for Python (nothing before that). Also, I tried resetting identity and different circuits. But every time it was offering contents in my language (along with English).

How is it even possible? Is it a bug? Or did Google perform a correlation? Or did webrtc/dns leak (Tor shouldn't have this issue)? Or cookie/javascript issues? Or anything else?

Answer 1

Open up developer console on your browser, run this

console.log(window.navigator.userLanguage || window.navigator.language)

If it prints your preferred language, that means you're fine. Otherwise, either your DNS or outbound connection leaked your real IP. One of the real case published here.