Is the localized extras supposed to be languages?

What is it exactly?

It depends. In some languages it's just the manual it would seem, but in others the guide and map as well, depending on where which extra was localized.

Well the language would have English at default so I'm not quite sure what it means for me as someone who purchased in English country. Is there any more details. I tried searching google and found nothing.

Your location does not matter to GOG, everyone is getting the same game and extras no matter where they are for most of the time (there have been issues with people registering as Australian).

The "extras" are game guides, the manual, maps, and other extra material and content normaly found with the game. "Localized extras" are those same things, but translated into Polish, German, Russian, Hungarian, Chech, French, Italian, Japanese and Mandarin. You do not have to download them if you don't want them, and they are not any form of content that you don't already get in English. If, on the other hand, you'd prefer to read a Japanese or French version of the manual, then be sure to check them out.