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A) "Press OK" is generic.
B) It rarely (never?) says "OK" when this command pops up.
Again, "Select" is boring.
As opposed to "KILL" indicating that you GOT A KILL on this enemy.
These are statements about your opinion on something.
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If it's grammatically correct, and in English, then it's an English phrase. I have to ask again, do you know what Engrish is?
You're saying then, that "Get Ready!...Engage!" IS an English phrase. This is true, but it's not very common...probably not what a native speaker would say. More common is Get Ready!...Go!.