How can I enter a custom maintenance mode message for a non-English D8 site?
This function should have worked as expected. There is not some feature somewhere else or another way this should have been done. The result described is the consequence of this bug in the Drupal 8 core. As of this writing a patch has been proposed but not settled on. Such are the consequences of playing around with beta releases!

In the mean time the only way to may a D8 site behave is to hack it using one of the following methods:


Manually implement one of the patches described in that issue.
Manually override the core template file (core\modules\system\templates\maintenance-page.html.twig).
Use the Interface Translation module to access the internal localization strings. Override the default maintenance mode string for the target language.


Eventually when that issue is closed and another release cycle happens it should be fixed and providing a custom message from the Admin page will work again as expected.