Earlier quoted context omitted.
The language button is right at the top of the page. If anything its more accessible on mobile?
Well, it's secret on mobile. I had the same problem - I wanted to switch languages on mobile wikipedia, but the page appears to provide no way to do that. Functionality that has been carefully designed so you won't find it isn't really an improvement over functionality that isn't there.
The button is labeled 文A, which is quite common for translation service, and is just under an article title so it’s literally the most visible UI element. It’s difficult to make it more accessible than that...