Hi, the languages you will need for Mac OS X are English, and on top of that whatever language you will be using your Mac in, as simple as that. If you only use the system in US English, and don't know a single word of Finnish, Korean etc, all those localizations are only taking up space, and will never be used (unless you change your OS X language to those, or create a new user account for your friends or family who will then be using that Mac in that language). Every single application has English language files, so you can tranquilly remove all otehr localizations if you don't need them.
That said, I do need to keep a few localizations for debugging and work related purposes, but I still leave only 4-5 in total.
