Extensions are made available for use by site by special Perl modules that are part of SPWS, so simply putting interpreter for some new language under correct name inside /extensions/engine won't make it available for use by site automatically. Please contact us if you really need some new extension to be added to the set of supported by SPWS. However, if extension is already known by SPWS, one can create an implementation of extension for other platforms not currently supported by SPWS.
The list of extensions to initialize is specified in the /site/config/extlist.conf directory, see of its syntax the introduction to extlist.conf for more information.
In order to be able to comfortably develop using some particular extension (e.g. PHP), a development version of extension should be used. Documentation on each extension suggests where to download development edition of extension.