First, you will have to check that the locale is available within the iOS system settings. iOS currently supports two tiers of locales, and a locale must be part of one of these two lists in order to ship on Mozilla products for iOS. Unlike Mozilla products for Android, there’s no locale switcher included in the app, so it’s possible to ship only locales that are supported by iOS.
To tell if a locale is supported or not, take a look on an actual device, under the Language Settings of the most recent iOS version available. Tier one locales are the ones listed under “iPod/iPhone/iPad Language”, and Tier 2 locales appear under “Other Language” (or “Add Language” if there are already some languages under that list). If a locale doesn’t appear in any of these lists, then it is currently not supported by iOS.
Once you’ve determined if the locale can be added you must go to Pontoon to update the list of current locales. If the locale is not available in Pontoon yet, consult this document for instructions on how to add it.
Once you have added a new locale in Pontoon, there is nothing more to do in order to get the locale into Nightly builds. This should happen automatically within 24-48 hours. To check that the locale is indeed now set up in the l10n repository, you can check that its correspoding locale folder has been created here for Firefox for iOS and here for Focus for iOS.
To check that the locale made in into the code repository, there is a script running (note that this method may not always be 100% reliable): here for Firefox for iOS and here for Focus for iOS. A more reliable method would be to look for the pull request that imports translations in the code repository, and verify that it created files for that new locale.
The locale will then ride the trains to release. The iOS release calendar follows closely the Android and Firefox desktop calendar, which can be found here.
Once you have removed a new locale from Pontoon, the existing locale folder then needs to be removed from the l10n repository, and all files should also be removed from the code repository.
As a mozilla-l10n code owner, you should be able to delete this folder yourself from the mozilla-l10n repo. This should be done via pull request, so we can keep track of the removal, and in case we'd want to reverse that action.
Then, file an issue on Github (here for Firefox for iOS, and here for Focus for iOS), so that the mobile team can work on removing their corresponding files. Once the issue is filed, and if you don't get any traction after a few days, you can ask for help in the
#mobile-xfnl-leads Slack channel.