Multilingual Capabilities of OSF SearchAPI
The OSF SearchAPI does support multilinguality without having to configure anything. What it does is to detect the language currently used by the user that perform a search query and will use that language to send the Search query.
The only think that is necessary is to make sure that the OSF Search web service endpoint does support that language.
Multilinguality and Search Templates
When will come the time to create the search templates for a multilingual Drupal portal, the Drupal themers will have to take care, and make sure they properly manipulate the entities to use the proper language to display in the search resultset.