title: Full text search index on a multilingual column
body: <p>I have a DB in which there is a table <code>shows</code> with a multilingual column <code>title</code>. I would like to optimise full text search by adding an index like so:</p>
  
  <pre><code>CREATE INDEX title_idx ON shows USING gin(to_tsvector(title));
  </code></pre>
  
  <p>I get this error:</p>
  
  <pre><code>ERROR:  functions in index expression must be marked IMMUTABLE
  </code></pre>
  
  <p>It basically asks me to add the language parameter to make to_tsvector immutable. the result would be:</p>
  
  <pre><code> CREATE INDEX title_idx ON shows USING gin(to_tsvector(LANGUAGE, title));
  </code></pre>
  
  <p>where <code>LANGUAGE</code> would be one of my target languages. </p>
  
  <p>Is it possible to create an index that works for several languages ?</p>