Sorry if this is already documented somewhere. I found this page but it's about policies not procedures.

As I learned recently, when a new language is added to civicrm-core:master, future PR's against the RC or current release will fail test runs in a confusingly unrelated way. So at least at the moment one of the non-obvious steps is that the language needs to be backported. (UPDATE: not strictly needed now as of 2020-04-08.)

So while it's fresh in my head trying to gather the steps.

ref: #4 (closed)

• Create the new translation of CiviCRM in transifex.
• If a language variant, sync it from another variant (jenkins script).
• Update conf/distributed_languages.txt.
• Wait until the language appears in the daily l10n tarball.
• Update xml/templates/languages.tpl, and then run bin/regen.sh which will update
  • install/langs.php
  • sql/civicrm_generated.mysql
• Make a core PR against master including those three files, and an upgrade script to add the option value.
• Backport the PR to the RC and current (maybe - pending https://github.com/civicrm/civicrm-core/pull/17021). 17021 was merged so it's not strictly necessary - lang will show as the code, e.g (nl_BE), and is technically usable, until master becomes current.