I'm not certain that the function you need is accessible at the FM end (i.e. through a pdfmark statement that can be included in the FM output stream). You can set this in the generated PDF prior to distribution using the Acrobat 9 Pro preflight capability "Set document language entry".
See this thread for similar info: http://forums.adobe.com/thread/285507
You might check with Shlomo Perets at Microtype, http://www.microtype.com
Microtype's Timesavers product has innumerable ways to manage PDF features from within FrameMaker, often using .ini files and/or markers that get morphed into PDF features.
Shlomo would undoubtedly know whether the language setting in PDFs could be controlled, either with the current Timesavers version or perhaps as an added feature.
There is a pdfmark route for setting the language, so it might be possible to define this in FM.
The pdfmark syntax for this would be:
[/Subtype /Document /Lang (en-US) /StPNE pdfmark
This pushes the structured element (/StPNE bit) for the document language onto the stack. You could try creating a postscript text frame on the first page of your document and insert this statement with the desired language code in parantheses, i.e. replace the "en-US" bit with what you need.
To do this, use the text frame tool to place a text frame somewhere in the margins of your first page. Next enter the above statement with your desired language code. Then, select the text frame (crtl+click) and use the Object Properties to change the text frame type to a "PostScript Code" in the Flow section of the properties.
This should do the trick at the FM end.
FWIW, this is the info from the PDF Reference manual on the /Lang identifiers:
Language identifiers can be based on codes defined by the International Organization for Standardization in ISO 639 and ISO 3166 (see the Bibliography) or registered with the Internet Assigned Numbers Authority (IANA, whose Web site is located at < http://iana.org/ >), or they can include codes created for private use. A language identifier consists of a primary code optionally followed by one or more subcodes (each preceded by a hyphen). The primary code can be any of the following:
• A 2-character ISO 639 language code—for example, en for English or es for Spanish
•The letter i, designating an IANA-registered identifier
•The letter x, for private use
The first subcode can be a 2-character ISO 3166 country code, as in en-US, or a 3- to 8-character subcode registered with IANA, as in en-cockney or i-cherokee(except in private identifiers, for which subcodes are not registered). Subcodes beyond the first can be any that have been registered with IANA.
Although language codes are commonly represented using lowercase letters and country codes are commonly represented using uppercase letters, all tags must be treated as case insensitive.