The Lord will say "Let it be", and all will be changed.
You really have to go with the idiomatic and historical (and thus in this context expected) translation not the verbatim translation. That means for this instance of Latin Fiat!, you would choose the English Let it be!
Notice that the Portuguese translation isn't really especially "verbatim", at least from the Latin. (I have no Hebrew.) It's a third-person imperative of an intransitive pronominal/reflexive verb with a sense that better corresponds to "become". But it doesn't make sense to translate it that way because that's not what it's always been translated to in this context. And it isn't literally commanding the light to make itself, either. :)
It isn't just Portuguese that does this same thing. Spanish bibles have historically translated the Latin Fiat lux into Hágase la luz, which you'll note is the very same approach that Portuguese takes. It just isn't how English bibles have done it.