I am truly hoping SOMEONE has an answer for this! I realize your post was some time ago - in case this went "off forum" here is the background.
In LiveCycle Designer ES, you can set the default "Form locale" under "Defaults" on the File Properties page. This is essentially the default language for the form. The problem comes when you generate a pdf from the form. Regardless of this setting, if you view the Advanced tab under the pdf's document properties, the field "Language" under Reading Options is blank, and greyed out - you cannot change it. This is the field Adobe uses to check for accessibility- if it is blank it gives an error message upon an accessibility check reading "the form lacks a language specification". I have searched all over this forum and all over the web trying to figure out how to correct this with no luck. Any pointers????