It looks an old instance, the time languages where not installed but included in the zip.
Editing the config.php file works but is a workaround.
When upgrading old instances I do prefer to have it the normal way and not use the disabled languages.
So:
1- Make all languages enabled on the language users config
2- Make sure there is no other disabled languages in config.php file
3- Download and install the languages that show up on the login menu (use Module uploader) so they can be managed by the language installed screen
4- Uninstall the languages I do not need (so there is no need to hide languages in the config file)
This way they you get a clean language files, no unused translations and the system works fine.
If in the future you will need an extra language I’m pretty sure you will need to install a recent language pack as translators do not sleep! 