The language is detected automatically! Just write the post in Norwegian 
. I think we may be able to change some text to make it a bit more obvious.
There are three states once you hit post:
- The non-English transiently appears (and is emailed out)
- A Locale-detector agent identifies the locale and attaches it to the post. This is not currently possible for anyone to do manually, unfortunately (cf. Posting in a disabled language - #10 by awesomerobot - Feature - Discourse Meta)
- The Translator agent adds an English translation
Once the locale-detector sets a non-English locale on the post, then you can manually edit the translation (and even the detected locale) with the multilingual button on the post toolbar (hidden behind the …s).