I'm really no expert on the Lucene.NET
search engine so I don't know what can affect search beyond the things that I specifically coded. I coded it to store and maintain the view roles with the search results in order to prevent users from seeing results in the search that they should not be able to see. Its very straightforward and I don't see anything in the code that would be affected by culture.
I don't know whether the web.config culture settings have any affect, the culture setting is applied to the executing thread.
I suspect the main problem is the lack of support for the Swedish language in Lucene and the solution is to implement support (someone may be working on it already)
The only other experimental suggestion I can think of is back up the contents of the Data/Sites/1/index folder then delete those contents and do a search. This will rebuild the search index. It may not make a difference but its possible that it will.
Wish I had more help to offer,