Showing untranslated content in taxonomy

I'm putting up a personal site for myself, and I usually write in either Norwegian or English. I find many blog posts that indicate that untranslated content is common to show, but I don't understand how people do it. Moreover, I add some complexity due to taxonomy use. I hope someone can find my use case below interesting and can help me out!

I have some entity translations, but content I write in either language, and I have enabled a multilingual site, so that those who want only English, or Norwegian, but most Norwegians would be fine with either. Consequently, the front page should consist of content from either language. How do I do that?

Then comes the taxonomy along. I'd like to blog about anything from my kids to the research I do with Semantic Web technologies. PlanetRDF readers are not so much interested in the former, and they are mostly English readers, so I'd like to have a list of content that is suited for them. Typically, that means that the content list (and the associated RSS stream) would need to contain content in English from any subcategory of "Semantic Web".

A local user group here in Norway, OTOH, might be interested in the posts I do in both English and Norwegian to any subcategory of "Technology". So, here, untranslated content has to be shown regardless of language.

I would prefer if I could just generally use the Accept-Languages header to decide which posts to show in a list, but I suppose that's rather harder to do, so I'll settle for a solution that can show all the untranslated content.