title: Filter many to many set from models instance
body: <p>I have a product model that has tags, the tags can be in multiple languages. when I get a instance of the product, i have a <code>product.tags</code> manager.</p>
  
  <p>I was wondering if there was a way to filter the tags connected to the product <strong>instance</strong>, that when I pass it to the <strong>serializer</strong>, i would only get the tags in a single language with the serializer output.</p>
  
  <pre class="lang-py prettyprint-override"><code>class Product(models.Model):
     ...
     tags = models.ManyToManyField(Tag)
     ...
  
  
  class Tag(models.Model)
     text = models.CharField(max_length=32)
     language = models.CharField(max_length=2)
  
  
  class ProductSerializer(serializer.ModelSerializer):
     tags = TagSerializer(many=True)
     ...
  </code></pre>
  
  <p>I am able to filter them manually then add them to the data response, like so:</p>
  
  <pre class="lang-py prettyprint-override"><code>tags_query = product.tags.filter(language=lang)
  tag_serializer = TagSerializer(lang, many=True) 
  </code></pre>
  
  <p>but I was wondering if this can be done through the serializers?</p>