In theory, we could "overwrite" all the other messages from other locales with an English version. Or adding a custom message without specifying the local that it would always show up on the only language available:
Set-AdfsGlobalWebContent -SignInPageDescriptionText "In case you don't read German, here is some English stuff"
Here is the result on a German browser:

But that's an odd thing to do. What is the issue with presenting the message in the local language?
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