Ok, but it is still an inconsistency, for newly created user fields the description exists for all installed languages, all description changes done in the admin.php?user-fields edit screen are reflected in the specific phrase for all languages. For the existing user fields (e.g. facebook) only the english language is adjusted. This doesn't make sense and is misleading in the case the ACP language is not english.
The value you define with a field is the default value that applies to all languages. You can choose to override this in specific languages. This is an action that has to be taken explicitly and that is what has happened here. (It would have been taken by the language pack author, but it's still explicitly decided as the action.)