Returns undef if no singular form exists, otherwise returns the
singular form of the topic
I18N - Only apply plural processing if site language is English, or
if a built-in English-language web. Plurals
apply to names ending in 's', where topic doesn't exist with plural
name.
Note that this is highly langauge specific, and need to be enabled
on a per-installation basis with $Foswiki::cfg{PluralToSingular}.