You should not mark as translatable "Default:" or "DefaultChoice:" fields, unless explicitly needed (e.g. default country, default language, etc.). If this Default field really should be translated, you should explain translators how they should translate it by using comments or brackets. For example:
# Translators: Default language name, but not translated _Default: English
_Default: English[ Default language name, but not translated]
Note that in the first case, Lintian ignores the comment unless it explicitly references translators and it is appears directly before the field in question.