Description of the need
I've just implemented hook_theme_registry_alter() in a custom block and checked the registry content to see long list of variables, including for unused layout templates. Disabling them on admin/structure/layouts/settings freed registry from unnecessary content and even-though human eyes might not notice, definitely made things slightly faster.
Proposed solution
I believe Backdrop must be shipped as performant as possible, so I suggest to disable all unused layout templates by default. Users can easily enable them if they need additional templates.
Additionally, we could add a pointer link on pages like admin/structure/layouts/manage/dashboard/configure as shown below:

Recent comments
Go to the account edit of the desired user. On the horizontal tab below "Region and Language," select "English" or another language. WFM.
Allow admin to select admin language seperate from front end language (multilingual)
@olaf - Sorry, but I don't think that works. I tried it and you are correct, with this change, I can switch from the English version to the German version of a page, without changing the entire...
Allow admin to select admin language seperate from front end language (multilingual)
Use case is an English speaking support person working on a multilingual site and fixing bugs with the French translation of the content. I found one solution for your use...
Allow admin to select admin language seperate from front end language (multilingual)