I have created a tagged document and set the reading option language to English and it passes a full accessibility check. I also want to post the individual chapters so I have split my document by bookmarks. All of the tagging and reading order is retained but the language setting is blank and the individual files fail the accessiblity check. The whole project creates 176 smaller files so resetting the language for each is not an ideal solution.
Sorry for the delay in responding.
I don't think there is any way to make the default language for the created files.
You can create a Fix Up that Sets the Language and then Make an Action that will run that Fix Up on all the files in a folder. Fixups are in the Preflight dialog.
I was able to do it in less thant 10 minutes using these instructions -- and what I already knew about creating an Action.
How to create a Fixup -- http://help.adobe.com/en_US/acrobat/X/pro/using/WS7E33E298-08DF-486f-83F7-A425C36B03D0.w.h tml