Question about Balkan languages - language people, I need your expertise!!

I read that a lot of Balkan languages are similar, in the same way that spanish spoken in Mexico, Argentina, and Spain are different but fundamentally the same language. Is this accurate?

I plan on traveling around the balkans next year and I hate being totally clueless so I'd like to know at least some basics. I speak decent spanish and I can confirm - if you learn spanish in Mexico, and then travel to Argentina you will understand what is being said despite some differences in accent and structure. Same with Cuba, Spain, etc. 10,000x better than knowing nothing.

So is it the same with Balkan languages? If I learn some Serbian will it be similar what is spoken Croatia, Bosnia, etc?

Thanks, - s