This documentation has moved in-tree: http://searchfox.org/mozilla-central/source/build/docs/locales.rst. Those docs are owned by the l10n team and should be up-to-date.
- The other docs don't specify but I wonder if you need to do non-artifact builds.
- gradle builds were failing when I had a multi-locale (and non-artifact) build configured
For older documentation:
- View revisions of this page older than 2017/10/3
- The original guide on :rnewman's blog