Directories in which to search for additional message catalogs (see
:confval:`language`), relative to the source directory. The directories on
this path are searched by the standard :mod:`gettext` module.
Internal messages are fetched from a text domain of ``sphinx``; so if you
add the directory :file:`./locale` to this settting, the message catalogs
(compiled from ``.po`` format using :program:`msgfmt`) must be in