I found a weird issue which may very well be the one you are also facing. I had the same problem: perfectly working in base language (English) but in German, Siri would just read the "default" responses (such as "Ok, done" or "Something went wrong" in German), but not the ones I specified in the localization file.
(One important difference: I don't have any parameter confirmation dialogs, but I do want Siri to mention parameters in the response.)
First thing I noticed is that removing the parameters from the response is a kind of workaround - Siri will read out the specified localization properly. Of course, this is not really an option, but interesting nonetheless.
The second, even weirder workaround is as follows:
add a new language locale to your project ("German (Germany)", you'll find it under Other). Uncheck all localization files (so nothing is duplicated), except the Intents localization. Use the existing German localization as a basis for the new language.
in the new localization file remove all parameters from all responses (I know this is theoretically wrong, but try it)
The result is that when you set your Siri language to German, Siri will read the responses from the original German localization (ignoring German (Germany)), with parameters, correctly. This happens even when you set Siri to German (Germany) and even when you set it to German (Austria) - does not matter.
I cannot see any explanation for this other than an iOS bug. For reference, I can reproduce this on 14.4 and 14.5 beta and I am reporting it to Apple. Will update here if I ever manage to resolve this "properly".