Languages on a WordPress project
We have a WP multi-site with two distinct languages, it doesn't really matter what those languages are, but in this case it was English GB (:uk:) and Welsh (:wales:).
We tend to develop custom code via mu-plugins, this means we can often have a lot of mu-plugins but we would like to share a text domain. This is because we have a single client, all of the mu-plugins are for this client and thus it makes sense to have a single text domain for all of their custom code. This is analagous, from a text domain point of view, to if you just did everything within a single theme. Similarly, we'd use the same text domain for the custom theme we have built for the client.
So the setup I'm trying to achieve is as such: