Comment 3 for bug 1613709

The problem is that valid formats for that field can be controlled by a regular expression in a org. unit setting. We can't really factor for sites that customize that setting, so the default string should match the default setting.

If any site customizes that setting, and this string is made translatable, then they can provide their own translation that contains a format matching their regular expression. Even if your OPAC is in English, you can provide your own translations to override/replace the default strings. It's not what most sites do, but it is one way to do it.