My browser states that I favour English, then French. My user profile on the website has "English" as language. Yet, when I get to the homepage, it tries to guess my language from my IP. NOOOO.
It could be system - but there is a surprising amount of people who set their OS to some language they are learning, or just English for convenience. And they do not want this to affect the web.
It could be the browser, but YouTube and most other services for that matter, are mostly used through apps.
It could be in the profile (but which one Google, YouTube, local app?)
This seemingly simple problem has an issue with discoverability.
Date formats, start of the week, 12/24h clock, auto translations, localised search results, language detection, it is all rubbish and clearly US-centric.
For all their (former) diversity dances, that company has very little to show for it.
However i haven't noticed this to be a problem with Google in general. All the Google products i've been using have been properly localised.