In T276140, the language switching button was moved to the bottom of the page.
There is no good reason to put it there.
It should be at the same place as it is on all other articles. T276140 says that main pages don't have headings, but other than that, they are mostly the same as articles. The placement should be consistent. This consistency consideration alone should be a reason to move the button to the top.
In addition, the main page is usually the most popular page in any given wiki. It is the page that user arrive to when they search for "wikipedia" in their search engine. However, the search engine often doesn't give the user the main page of the Wikipedia in the language they wanted, and the user wants to switch to another language. In this very frequent scenario, switching the language should be easier, not harder. This is another reason to put the language switcher at the top.
Not having a heading is not a problem. Just add some empty space without any text. Or maybe put the word "Wikipedia" there.
T276140 also discusses how main pages in some languages have lists of some other languages near the bottom. It's true, but is this actually a good design? To whom is it useful to see them there? To whom is it useful to see them sorted by the number of articles? It may be interesting to some Wikipedians, but most readers don't need that. Most readers want to find a language in which they can read.
So please, put the language switching button on the main page at the same place where it is on other wiki pages.