Refactor code to make use of make_plurals_sql_fragment

[Henning Eggers]

The translation message table has six seperate fields to store translation strings. In SQL queries this often makes code in WHERE clauses repeat itself six times. To make this easier and provide for an easy change of the number of plural forms (hopefully won't happen) the helper function make_plurals_sql_fragment was created.

The code should be checked to find places where this function can be used and those place should be refactored to do so.