Where did you make the changes?
In the file core/components/babel/src/LanguageSubtagRegistry/language-subtag-registry or in the cache file core/cache/babel/languages.cache.php?
Maybe you have to delete the cache file, to see the changes you made to language-subtag-registry?
It could also be wiser to use a custom template (&tpl) that outputs the correct strings based on the cultureKey placeholder, than to ‘hack’ a file in the code of the Babel extra.
I had made the changes to core\components\babel\vendor\iana-language-subtag-registry\language-subtag-registry. This worked in the past.
When doing it to core/components/babel/src/LanguageSubtagRegistry/language-subtag-registry and deleting core/cache the desired result shows up.
Thanks for your prompt support!