I am looking for a Joomla 2.5+ component to edit the language files (ex: languages/en_GB/en_GB.ini).
Changes have to be made from the administrator. We know we can edit the underlying files, but that will require change management in our production environment, with all the associated redtape.
I have tried searching for a solution on Google but didn't find one.
You don't need for Joomla! 2.5 the functionality already exists.
In Joomla! 2.5+ you can use the built-in language overrides editor. Simply select Extensions->Language Manager->Overrides and click the "New" button.
The nice thing about doing it this way is that you can safely apply Joomla! core updates without losing the overrides you've added.