editing language keys or adding new keys is done through Language Management, in administration. You don't need to modify the xml files.
In language management you can edit your translations inline, even by comparing more than one language at a time and editing all of them at the same time.
That said, to ADD a NEW language key you just need to
- click on ADD, insert the module name and the key name (normally it is _WRITTEN_LIKE_THIS)
- repeat to do the same in other languages
To EDIT an EXISTING language key, just click inside the translation cell and write your new translation.