When you use issue.get("Field Name"), it looks for the field by ID and by name. But if the current user is using a non-default language, it will look for the localized name (in the user's locale) and not in the system's default language (which is likely to have been used) nor in the English locale (which is what is documented in our documentation).
When passing a field name (not ID), which should look for that name in:
the system locale
the English locale
the user's locale as a last resort (mostly for backward compatibility purposes)