Add checks for locales and locale-related issues
Some Steam games assume that the en_US.UTF-8 locale is available, and it's reasonably likely that others assume that the C.UTF-8 locale is available, or that the locale environment variables are set to usable values.
Having checks for locale properties will also help us when experimenting with containers: we can check whether the locale that was set outside the container is available inside the container.
Related to https://phabricator.collabora.com/T13116 and https://phabricator.collabora.com/T16197.
/cc @denittis @jpwhiting