Currently, we have the 'locale module' -core-, which deals with language management and interface translation and 'i18n module', which provides support for multilingual content, translations, but also some language management, providing a block for the users to select interface language.

This is some mixed approach, that is somehow confusing, causes some overlapping and  wouldn't be there if we had some clean off the ground implementation of multilingual features in Drupal. My proposal would be in the line of provide this kind of implementation, with some support in core, and some lightweight modules managing very specific parts of the whole 'multilingual' system.