[kh99]

If you make a user an administrator, you can then go to "Administrator Permissions" under Usergroups in the adminCP and select which permissions they have. There's one for languages so I suppose if you gave someone only that permission then they would only be able to edit phrases. I'm not sure if there's any threat to the forum beyond what someone can do with phrases, maybe someone else can comment.

Also, I'm not an expert on configuring users and groups, but I think you can give someone an admin permission without having to have them look like an administrator (maybe by adding admin as a secondary group? I'm not sure).