Can I use translated properties for URL formats
This turned out to be easier than expected. It's not documented in the filters section of the docs, but the t() filter can take other parameters after the  tokenised variables you might want to replace in the string. Paraphrased from the source code of the globally available t() function:


source: Defines which message source application component to use. Defaults to null, meaning use 'coreMessages' for messages belonging to the 'yii' category and using 'messages' for messages belonging to Craft.
language: The target language. If set to null (default), craft()->getLanguage() will be used.
category: The message category. Please use only word letters. Note, category 'craft' is reserved for Craft and 'yii' is reserved for the Yii framework.


So, the solution was as simple as:

{type|t(null, null, 'nb')}/{slug} for URL format for Norwegian and

{type|t(null, null, 'en')}/{slug} for English.