I will set up collections - one for English and one for French, and add articles into each.
I suppose setting up the entitlement server would be useful if you wanted to restrict access to either English or French for specific users? Would this require an internet connection each time a user tried to open the app on the device?
Using keyword on Collections would have similar reasoning as I describe above. No reason I can think of not to use keywords of either type on a collection. But I don't think this is a real way to implement language support. Using separate collections for English and French is only organizing content.
Preflight would allow testing of keyword use in card mapping and your organizing content just fine.
Collections are not dynamic, so the won't populate based on keywords or other metadata.
You could build collections for English and French and put corresponding articles into each. Then readers could choose to look into their preferred collection, or you could sell/restrict access to one or the other.
A note on keywords -- eventually these may be available to users, their potential use in building a dynamic collection or in search are possible uses sometime in the future. Whereas internal keywords will not be visible to the user, so these are better used inside of DPS 2015 by you.
Both types of keywords are good candidates for use in mapping rules -- this controls which card design displays the card. So an article with an internal keyword of "English" might get one card design, and "French" results in a different card design. That would provide visual identity to content with those keywords or internal keywords.