Warning: this is my understanding based off of how the queues work with Frontlines and PvP.

First off, each Duty has a different queue set, each queue set is subdivide into 4 "language" sub-sets and each sub-set is subdivided into parties.

The 4 "language" sub-sets are J, E, D, and F. By activating/deactivating the language selector you are determining which language sub-sets you are allowing the server to put you in.

Parties are determined by content type:
4 man content parties are [T][H][D][D].
8 man content parties are [T][T][H][H][D][D][D][D].
post-LotA 24 man content parties are [T][H][H][D][D][D][D][D] with LotA using standard 8 man parties.
Frontline PvP parties are [IF][IF][IF][IF][IF][IF][IF][IF], [MS][MS][MS][MS][MS][MS][MS][MS], and [TA][TA][TA][TA][TA][TA][TA][TA].

When you push the Join button in the duty finder what is really happening is that you are telling the server to put you in the oldest open slot for whatever you are queuing for that is available in your selected language sub-sets. If their is no open slot it will create a new party and place you in that party. Every so often (~3 minutes I think) the server will check if players have abandoned the queues and then rebalance to fill any holes using a oldest queuer oldest slot mechanism. Between these rebalances, new queuers are put into the oldest open slot.