update the list of i18n postgre stemmers

Just updated the list of languages for fulltext search. And changed the default stemmer to "simple" instead of "english" for unsupported languages.

There are however a few languages left that Postgres doesn't support out of the box (e.g. Czech). Still, users can add this support to Postgres manually, and then they would like to switch the used fulltext language. Is there a way we could make this a separate site setting?