Language codes are used as keys in the `locales` list in manifests. Language codes have to be either in the format "es" or "es-ES". I just had a partner with "es_ES", which broke localization in the app. Marketplace could validate the correct format for language codes.
Can you describe more about breaking localization? There is a whole world of variants out there past xx and xx-YY formats and I don't think we want to go down that road. (See http://en.wikipedia.org/wiki/IETF_language_tag and http://tools.ietf.org/html/rfc5646 for some reading to help you sleep)
We should validate to allow the values we support, I guess. The partner that it didn't work for used underscores, as in en_EN.
https://github.com/mozilla/app-validator/commit/3e595d39de67e285dba6bd18b4b9bc1e28126b5e We flag unknown locales now, but this points out the problem.