Only the default identity attributes are translated. This is because there is an i18n translation map for the default attributes. Custom attributes aren’t in this i18n translation map and therefore have no way to be translated based on the browser setting. This would be a new feature request to be able to translate custom attributes. This is the closest idea I have found in the ideas portal. Please add your requirements and upvote: https://ideas.sailpoint.com/ideas/GOV-I-1562
Alternatively, if you feel this should be a net new idea, then you can submit a new idea to the ideas portal.