Right now if one specifies 2 language variants for his site such as Swiss German (de_ch) and High German (de_de) both are shown in language selector in dialog as "German" only.
This is because info.magnolia.ui.dialog.formdialog.ItemFormView uses only language part of locale to display users what language they can choose from at

public void setAvailableLocales(List<Locale> locales) {
...
languageSelector.setItemCaption(locale, locale.getDisplayLanguage(MgnlContext.getLocale()));
...
}

The solution would be to include country code in the caption as well.