How to set Language in Document properties via batch processing

You might want to try out Preflight (look for its menu item in the Edit menu, or use Cmd-Shift-X to open Preflight).

In Preflight, create a fixup (switch to 'fixup' mode by clicking on the right most of the three icons above the list in the Preflight window) that sets the document languege to the desired value. In order to create a fixup, use the fly-out menu in the upper right of Preflight and use "Create New Preflight Fixup". Once the resp. dialog opens, use the search field an enter "language". You will find three possible fixups, and you will want to use the one called "Set document language entry". Set it up to meet your needs. Make sure to provide a useful name to the fixup and save it.

Next, create a profile (click on the left most of the three icons above the list to switch Preflight to 'profile' mode), and create a new profile. In that new profile, assign the fixup (created above) in the "custom fixups" panel for this newly create profile. Give the profile a useful name and save it.

Now, you can use the "Action Wizard" that call the Preflight profile just created.

An alternative option might be to create a Droplet (see fly-out menu in the Preflight window, use "Create Droplet", make sure to first select the just created profile in the profile list). Save your droplet to disk (setting it up to match your needs), and then simply drag PDFs  onto it.

HTH.

Olaf