Display a block depending of the localization of the user

I need to display some blocks only if a users is in a specific country.
In order to do that, I would like to add a new item to the blocks menu where a user can configure the countries for a specific block.
But I wasn’t able to override the block’s menu or manager files. Can anyone help me with that?

@nathalie.lot This should get you started: Override (almost) any core file in 5.7