I've talked about this before. I'll talk about it again. So often I find depictions of Spock that are nearly accurate in works of fiction. I've read a handful of Star Trek novels and one of the most common errors in his characterization is this: they assume he has no knowledge of the rhetorical devices of the English language. Jim says 'cool,' Spock looks on confused until it's explained to him that it means 'impressive' rather than 'cold.'  

But, Allison, you say. Spock's from Vulcan. He speaks Vulcan. ESL learners often struggle with idioms, because there's no way to find their meaning through the words themselves. Terran idioms and manners of speaking aren't going to be familiar to Spock, so of course he's going to be confused.

To that I have two words: Amanda Grayson. 

Never forget that Spock's mother was human; never forget that Amanda Grayson was not the kind of woman to turn her back on her own heritage to merge more seamlessly with her husband's. I'm certain Amanda used Terran phrases at home, and I'm equally certain Spock picked those up. He doesn't use them because they're too informal; they don't mesh with his style of speaking or thinking. He understands their meaning, though. 

And if he doesn't? He'll attempt to figure it out through context, and if he can't do that, he'll ask. Assuming otherwise, assuming that he's always confused and unable to comprehend the way his crew-mates speak, is an insult to his intelligence. Spock is smart, and Spock is adaptive. He won't compromise himself -- which is why his English isn't casual -- but he understands the cultural differences between planets. Once he's been told what something means, he won't have to ask again. 

Federation Standard -- it's English, or at least, heavily based on English. (This, of course, is because Star Trek was geared towards an English speaking audience.) Even though Spock grew up on Vulcan, he learned English as he learned his planet's native language. He isn't coming to Earth and learning an entirely new language. He's already intimately familiar with it. (Note; idioms and other similar things would undoubtedly change from planet to planet, depending on that planet's culture.) He has a human mother, so he's also familiar with Terran mannerisms regarding the language.

And -- and this is most important -- by the time Spock meets Jim Kirk, he's been living on Earth for years. I cant't find my timeline (and I'm kicking myself for it) but I believe he declined admission to the VSA in 2250; that would make him twenty. He doesn't meet Kirk until 2258; that's eight years on Earth. Spock's had eight years to learn idioms, to learn colloquialisms, to learn the idiosyncrasies of how we speak. He's learned the same about other cultures, too; Starfleet doesn't only consist of humans raised on Earth.

Also, he's known Uhura for a significant amount of time before the first movie starts. If he's curious as to something's meaning, she'd be able to explain it to him. (He probably teaches her Vulcan, at some point, too.)

Spock's not going to be floored every time he hears an idiom. Most of the time, he won't react, because he already knows what it means. If he doesn't, he'll attempt to figure it out via context clues. If he can't do that, he'll ask someone, but he doesn't need it explained to him in a condescending manner. 

icons of Captain Pike for daretheuniverse

I faulted myself for the death of my mother, and for the destruction of my planet, despite there being no logical basis for these feelings. Neither could have been saved through an alteration of my actions alone, and yet I have found that I have created a hundred different scenarios in which I act differently and save Vulcan.

five years of roleplay experience; three on tumblr; nearly a year of writing spock himself. familiar with source material, both aos & tos. plotted threads strongly preferred. icons & gifcons are used, and askbox is always open for plotting. skype & kik available for mutuals.