This achievement and "O Daughter of Babylon" are mutually exclusive. They are the two different endings for the game, and one must be chosen for completion of the DLC. Read on to know when to save in order to refrain from a second playthrough of Honest Hearts.
After completing the "In a Foreign Land" achievement, you will get the quest "Gathering Storms." For this quest, you must talk to Daniel in order to decide the fate of Zion. Save before you talk to him! You can talk to him about the decision and still have the opportunity to back out in order to complete any side quests, so back out of the conversation and save your game. In order to get this achievement, choose the option to evacuate Zion. The achievement will unlock just before the ending cinematic.
If you're having trouble with this DLC, click here for an abbreviated walkthrough.