register_translation() - why required language?

[zopyx]

The signature of register_translation(language, obj) is a bit weird.

First the translated content object obj should have proper language property set. Having no language set is an indicator of bad or lack of metadata. register_translation() should take the language from obj.

What is the usecase for specifying a possibly different language here?

Regarding the documentation:

https://docs.plone.org/develop/plone/i18n/translating_content.html

mentions register_translation()

but the package's README does not.