When using a VPN to access Youtube, the adverts played to you will be in the local language of the VPN destination, yet Youtube can deliver the appropriate language content. Strange that!
How many advertisers advertising in a VPN destination where a particular language is dominant would advertise in another language? YouTube doesn't care whether you can understand it. Those advertisers may not be looking to advertise to people who can't understand their language either.
How many advertisers outside of that country are going to ask YouTube to play their ads in that country in a language different from the dominant language? Probably not many.
How many advertisers outside of that country are going to ask YouTube to play their ads in that country in a language different from the dominant language? Probably not many.