ChoiceTextGroup i18n.

**Ticket**: [BLD-726](https://edx-wiki.atlassian.net/browse/BLD-726)

@sarina please review.
If something shouldn't be translated, don't put it in a translatable string.

`_({input_type}: unexpected tag {tag_name}").format(input_type="ChoiceTextGroup", tag_name=self.tag)`

Translators comment should explain the meaning of "tag".