macOS Mojave, Dictionary.app: How to install dictionaries to offline computer?

They are located in (HighSierra)

Aside from this, you will find packaged dictionaries here:

~/Library/Dictionaries/

there is also the words file here /usr/share/dict/words

User specific Dictionaries are stored here /System/Library/Assets/com_apple_MobileAsset_DictionaryServices (based on your Language and Keyboard selection), here is mine, so if you have chosen Polish it will be there.

copy and paste in to Finder GoTo

Do not copy the Folder, just the dictionary it self.

Now in MacOS Catalina the folder is apparently /System/Library/AssetsV2/com_apple_MobileAsset_DictionaryServices_dictionaryOSX. There's also another folder called /System/Library/AssetsV2/PreinstalledAssetsV2/InstallWithOs/com_apple_MobileAsset_DictionaryServices_dictionaryOSX that might contain the dictionaries as well.

To make the dictionaries work with Dictionary.app, the manifest com_apple_MobileAsset_DictionaryServices_dictionaryOSX.xml in the folder is apparently crucial. It records the correct folder names for each dictionary.