Does Apple have an extended spoken content language library?

No, you can only select from the voices included. There is no extended library available. You can, however, submit feedback to Apple requesting such a library. If enough users submit feedback Apple may (or may not) add such a feature to future releases. None of the languages you mention are listed.

Product Feedback - Apple

You can check if they have the languages here:

1. Open System Preferences: Go to System Preferences > Accessibility > Spoken Content (or Speech).
2. Voices: Click on System Voice or Voices to see the available options.
3. Customize: Click on the drop-down menu and choose Customize to see and download additional voices.
4. Select Language: Browse through the available languages and select the voices you want to download.

patrickj123 wrote:

For example I want to know how to select and download voices for Irish, Basque, Koine Greek, Attic Greek, Classical Latin, Ecclesiastical Latin, Lithuanian, etc.

Basque is already available in Ventura.

Change the voice your Mac uses to speak text - Apple Support (MK)

Read the information in the link above for more on this.