Order confirmation message is not translatable in custom checkout

Problem/Motivation As @Berdir wrote in [#3228451], the order confirmation message can now be translated in the checkout flow. But this only works for the "default" checkout flow! /admin/commerce/config/checkout-flows/manage/default/translate/de/edit shows the confirmation message and allows translating it: /admin/commerce/config/checkout-flows/manage/my_custom_checkout_flow/translate/de/edit does NOT show anything translatable besides the label: So there doesn't seem to be any way to translate the checkout message for multilingual projects using a non-default checkout flow.

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Order confirmation message is not translatable in custom checkout flow [#3375464]

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