How does MongoDB text search work?

I create a new collection test_index and insert two documents:

var mydocs = [

{t: "Her", language: "english"},
{t: "El asaltante", language: "spanish"}

]

db.test_index.insert(mydocs)

      

        
        
        
      

    

Create a text index for the field t

:

db.test_index.ensureIndex({"t": "text"})

      

        
        
        
      

    

Do a search:

db.test_index.find({$text: {$search: "asaltante"}})

      

        
        
        
      

    

No results are returned.

Instead, if I use $ text like this, it works.

db.test_index.find({$text: {$search: "asaltante", $language: "spanish"}})

      

        
        
        
      

    

Is this an intentional use? Should I do $ 10 text searches if I have 10 translations. This sounds ineffective. How does MongoDB's $ text $ search engine work?