Swahili came back to her more quickly, English more slowly.
Indicating that (perhaps) Swahili is Uhura's native language, what she spoke as a child. But English was learned later, maybe in High School or after she entered Starfleet Academy.
What Nomad did to her might have been similar to a stroke, all the information is still there, but you have to relearn how to get to it.
But NOMAD itself said:
NOMAD: The unit Scott required simple structural repair. The knowledge banks of this unit have been wiped clean.
So NOMAD didn't understand what it did?--Which I'd have guessed was a cut and paste job with Uhura's knowledge the files. But it was clearly interested in what she knew, hence the "think about music" command. It wasn't using something it considered a weapon, I'd say, something that scrambled her for the sake of scrambling her. It was more like it was interfacing with another machine to gather data. But not copy and paste, cut and paste, if we can believe NOMAD itself.
NOMAD refers to her knowledge banks. I don't know about others, but I don't usually think of my first language as "knowledge," although I know that it is. I think of it as part of my inherent, day-to-day makeup, that is, speaking with other people, reading, etc, are so ingrained as instant habit that they're nothing like, say, learning a branch of math you never studied before, or learning 100 new songs. Could be equivalent to a computer's operating system instead of data files, and maybe NOMAD viewed such deeply ingrained parts of a biological unit's functioning as its operating system, separate from its "knowledge." So Uhura wouldn't have lost her first language (but almost all else).
