You can see using plane mode, the mic to use 'Dictation' is unavailable.
According to this question and to the screenshot below from my iOS app : Is there is anyone know which kind of Apple devices is allowable to use 'Dictation' in Offline Mode ?
As mentioned in the update in the post you linked: you can use supportedLocales()
to get the list of supported locales and also isAvailable
for the current locale to see if it's available.
The user has to turn on dictation System Preferences -> General -> Keyboard
. It's available in English but not for e.g Swedish.