As soon as you complete this mission, you can start interrogating Soviet soldiers.
Metal Gear Solid 5: The Phantom Pain - Extract Interpretor (Russian)
Head to the marked area on the map, and you'll come across a small building with a red door, with two soldiers patrolling outside.
The interpreter is inside. He might decide to stretch his legs, so whatever you do, don't kill the guy wearing a red beret.
As soon as you get eyes on him, he'll be flagged as the target, so there's little chance of you actually shooting him by accident.
Knock him out and fulton him back to Mother Base to complete the mission.