website: Modernize and simplify language logic

The website has really complex deprecated hugo logic for localization.
To simplify this and eliminate linter warnings, we first need a modern
hugo language key. Build that in hugo.toml, and remove all the extra
files related to the old way of doing this. This removes several INFO
messages about deprecated syntax during the build, increases clarity
of the build system, and allows us to begin transitioning off the
remaining deprecated elements.

Reviewed by: carlavilla
Differential Revision: https://reviews.freebsd.org/D56173