An elegant solution to this seems to be more complicated than I had hoped.
See sphinx-doc/sphinx/issues/8342.
Read the Docs does something to let you point to different projects for different languages and then have a language picker in their flyout (see their docs). But this doesn't seem to be part of core sphinx-doc or a theme.
there will be https://docs.opendronemap.org/installation.html and https://docs.opendronemap.org/sw/installation.html it would be nice to have cross links between languages. would this be... just links on the homepage to the other language homepages? is it possible to have it in the nav so you can switch between languages on any given page for that page?