I am afraid that there is no way to do it (however, please make sure to check the update at the end of the answer).
As mentioned at the Speech Framework Official Documentation:
Best Practices for a Great User Experience:
Be prepared to handle the failures that can be caused by reaching speech recognition limits.
Because speech recognition is a network-based service, limits are
enforced so that the service can remain freely available to all apps.
As an end user perspective, trying to get Siri's help without connecting to a network should displays a screen similar to:
Also, When trying to send a massage -for example-, you'll notice that the mike button should be disabled if the device is unconnected to a network.
Natively, the iOS itself won't able this feature until checking network connection, I assume that would be the same for the third-party developer when using the Speech Framework.
UPDATE:
After watching Speech Recognition API Session (especially, the part 03:00 - 03:25) , I came up with:
Speech Recognition API usually requires an internet connection, but there are some of new devices do support this feature all the time; You might want to check whether the given language is available or not.
Adapted from SFSpeech?Recognizer Documentation:
Note that a supported speech recognizer is not the same as an
available speech recognizer; for example, the recognizers for some
locales may require an Internet connection. You can use the
supported?Locales() method to get a list of supported locales and the
is?Available property to find out if the recognizer for a specific
locale is available.
Further Reading:
These topics might be related:
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