In hg 6353:9d209d2b34ae the value of the config
main.indexer_language setting is validated in
indexer_xapian.py.

A bad language setting results in an error when loading any web
page (and probably a failure from the email gateway).
It should probably be validated in configuration.py.

To do this, we need:

 1) determine if the xapian indexer is being used.
 2) determine if the language is valid by calling:
    xapian.Stem(language). A xapian.InvalidArgumentError
    indicates a bad language.
 3) report to the user the error and the list of good languages.
    See current implementation in backends/indexer_xapian.py.

1 is tricky. If [main] indexer is unset or set to xapian, we need
to test the config value. But there is no guarantee that
the [main] indexer definition will be available when we need to
validate the indexer_language.