Converting macos dictionaries to portable format
Dictionaries used to be in /System/Library/Dictionaries, but they were moved (c. Sierra?) to various subfolders in /System/Library/Assets/com_apple_MobileAsset_DictionaryServices_dictionaryOSX.

The format is a package, and the data files inside seem to be some non-human-readable binary data format, though they start off life as XML files.

Assuming you've searched for projects that can import Apple's dictionaries, rather than just exporting them, and found nothing; then the task may be 'non-trivial', and you may be better off using the very good open source DICT format files.