To be precise, these aren't different languages, just different dialects. The differences between Koine and Classical are not particularly significant from what I can tell, and more center on word usage than grammar. It's not until you get into Byzantine Greek that things really start to change, and even then it's gradual. I'm surprised to hear that with Spanish (although I don't know where you live). I'm in the southeastern U.S. and have used it in virtually every job I've had since high school. I also use it to talk to folks in the service sector pretty regularly (e.g. the guys that sometimes do our lawn).I didn't realize there were different Greek languages...
As far as I know, most of the changes from Classical to Koine Greek were phonological. Like you said, that doesn't matter so much if you don't need to speak it.
That's most of it. There are a few little grammatical things too, from what I understand. The 3rd person pronoun (αὐτός) is used more, and a lot of -μι verbs transition to regular ol' -ω.
I live in the Midwest. I'm sure I had friends who spoke Spanish at home, but they weren't open about it in person. It's very whitewashed up here in certain areas. I did move to an area after college that was heavily Spanish, and I could have used it then had I remembered any of it. I picked up certain words from the stores and stuff, but nothing to really stick with me. We really need to change how we view foreign languages here. We shouldn't be waiting until high school to learn a second language. But who decides what the second language we learn at the same time as we start 1st or 2nd grade will be?
Yeah. I was lucky in that I started in 7th grade (so about age 12), and I'm sure that helped.