They have to be tested in some way. The government has a word they used for the process but, I can't think of it now, too tired.
If I recall correctly, an FBI muslim agent refused to follow an order that involved his spying(for lack of a better word) on other muslims. Would you trust someone like him to translate documents correctly?
To exclude muslims from that positions the aclu would be screaming from the rafters.
Then why would Americans want to listen into conversations in arabic if they didn't understand the language? Too chaotic.
This is why ordinary citizens can be better at doing this than government employees. They are not constrained by politics or the constitutional limitations.