Sorry for the delay in response.
Please refer to the solution provided in this help document: Mac package certificate has expired for Adobe Acrobat or Reader
Let us know if you need any further information.
Thank you for the response, Meenakshi, but it does not answer my original question: Where are the updated Mac package installers for the Reader DC Extended Language Pack and the Spelling Pack? The article you linked instructs us to download new, updated installers, but the Adobe documentation I linked still points to old, outdated installers that fail upon installation. I don't know where else to look for these new installers.