How can I search in another language?
If you are using entity translation for translating the content, you will have to install the Search API Entity Translation module.


  Search API Entity Translation module provides multilingual support for the Search API module through integration with Entity Translation.


This module provides a new index type : "multilingual node index", similar to the "default node index" which is provided by default by Search API.
You can use this to create the views and they will honor the language selected from the language switcher.