(Separating locales for admin panel and forum seems to be related as well.)
The current state of things is a bit strange, and I am not sure what is feature, what is bug, and what is just inconsistency.
This is how the admin dashboard looks like when the default site locale and the personal locale differ:
The areas in red are showing the site locale, while the rest is displayed in the personal locale. Note that in some cases the same title has two different locales.
This is not the end of the world because it is likely that the admins will know the default site language and of course the personal language that they have selected. It’s just strange.