W3C defines formats for language codes at http://www.w3.org/International/articles/language-tags/, which is what Drupal refers to for language code formats as well. This does allow for numbers in language codes for variants (which was not yet supported in Drupal 8) but does not allow underscores (which still was). Drupal 7 and 6 referred to http://www.ietf.org/rfc/rfc4646.txt for language code formats, which did not allow underscores either. Therefore a significant enough transition period was allowed for older migrated sites to fix language codes. Drupal 8 dropped support for underscores alongside introducing a stricter validation against the W3C defined format.
In your Drupal migrations, instead of language codes with underscores such as en_gb, use en-gb.
