About Lingua Franca...

Deploy at the northern LZ. Wear the "Woodland" fatigues. Bring D-Dog. Start at dawn.

The "viscount" interrogation only happens at night. Thus, you will need to wait until night after completing the first three prisoner interrogations.

The scripts are fragile (or rather Kojima thinks you must do everything the Kojima way), and one of the easiest ways to break them is triggering a CP alert. Any state of alert, not even a combat alert, causes the failure of this task. Also it seems you have to identify all the prisoners (or at least mark them) using binoculars (get solid green markers on them). If you are noticed during interrogation scenes, the script also breaks.

The first prisoner you encounter at the start of the mission must NOT be extracted immediately. Listen to the conversation, mark him, and use him as a waypoint to the second interrogation scene. The first conversation is not an interrogation, but must be listened to.

After the interpreter and that prisoner split up, neutralize the guard near the watchtower and steal the jeep to beat the interpreter to the first interrogation scene. You can interrogate the watchtower guard to get the intel file location.

Listen to the first interrogation from a distance using binoculars. After the interpreter gets into the jeep and drives away, neutralize the guard and extract the prisoner.

Drive along the southern edge of the camp to reach the previously marked prisoner before the interpreter does. There will be two prisoners. Hide on the small cliff above the cages and prepare to listen. At this point, the script will enter the "one prisoner missing" state if you so much as get noticed, so avoid detection.

These two prisoners will be executed after the interrogation if you do not act quickly. As soon as the interpreter leaves and one of the guards approaches them, immediately neutralize the guards and extract them both. The time window between the interpreter leaving detection range and the guards killing the prisoners is very narrow. Extracting these prisoners reveals the primary target's field of movement. Mark its southwestern end on the map - this is the final interrogation scene.

There is a large shed containing a radio transceiver near this place. It contains an intel file. Scan it to get the last prisoner location. Go there and identify him using binoculars - it seems to be necessary for the script to work correctly.

You can now roam freely. Just make sure that by dusk you are at the southwestern end of the discovered FOM (which will disappear as soon as you identify the last prisoner). There are three guards there; deal with them in advance and wait.

At dusk, the remaining prisoner will start to be escorted to a building in that area. Approach it and wait. The interpreter will also start moving toward that area and will eventually enter the building as well.

Hide on the cliff above the building and listen. Do NOT hide under the window. If you get too close, the "Snake detected" script triggers, the prisoner collapses at the entrance, and the interrogation will not occur.

After this, the task should complete.