Hej @manuel
its not possible to have more than one search index. (source)
So that being said its not possible to show only one language without custom code (which will scan through all result links and check if there is “de” or “en” within the url).
You may can solve this by using the [[de]] tags for displaying different languages in certain fields. And maybe its possible to just add these tags in the Site-Search Description/Name fields of your Blog Collection Page.
Take a look here for the tutorial how to do it:
But i think the custom code should be pretty easy to write and implement. 