For the first half of the movie Anna speaks only Spanish and Poncho has to translate for Arnold. Then all of the sudden, mid-way through this flick, Arnold starts asking her questions and she starts to speak perfect English. No one is surprised or reacts in any way to this instant transformation.
When they first meet her, she's an "enemy" and taken captive. She refuses to speak English because she's being uncooperative, and likely is hiding the fact that she can understand them (which increases the likelihood they'll say something useful thinking she won't understand). After the Predator attack, she realizes it's in her best interests to work with the soldiers.
You're adding too much to this fiction story. Nowhere in the finished script or movie dialog does it even say anything about Anna's motives for switching from Spanish to English. Watch the lack of reaction from Arnold's men when she suddenly busts out with the English explanation of what the Predator is up to. Not even a raised eyebrow or a "What the heck?" A slip-up by the director.
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