define policy for adding, supporting, and revoking languages

[cfm]

Description

After multiple localization cycles, several supported languages have fallen below our stated requirement of 100% translation coverage. We've also received requests to add languages that may or may not receive enough translation coverage to be supported. We should update:

1. our policy for when languages are supported for release versus merely available for translation in Weblate (Update unsupported languages in Weblate #6156, unblocked by centralize list of supported languages #6387 and Fail gracefully if language no longer supported #6366); and
2. reconsider Weblate permissions for adding new languages for translation (unblocked by freedomofpress/infrastructure#3944).

Although I'm filing this in securedrop, I think it's likelier to be addressed as documentation in securedrop-dev-docs than in code here.

1. Draft policy
2. Consult with Localization Lab
3. “Translate” this proposal into a policy… → publish policy on supported languages securedrop-dev-docs#35
   • documented at developers.securedrop.org
   • linked from https://forum.securedrop.org/t/about-the-translations-category/16
   • linked from Weblate
4. Adjust and test Weblate permissions → freedomofpress/infrastructure#4120
5. Apply this policy to the current set of supported languages → [2.6.0] handle special cases for this localization cycle #6716

User Research Evidence

• https://forum.securedrop.org/t/reviewer-application-for-portuguese-portugal/1444
• https://forum.securedrop.org/t/starting-bengali-translations/451
• Private correspondence with Localization Lab

User Stories

As a maintainer and Localization Manager, I want to have and be able to communicate about clear standards for listing and delisting a language as supported.

As a volunteer translator, I want to have a clear path to adding a language and getting it listed as supported.

[cfm]

In today's Server hangout: confirmed for v2.6.0.

[cfm]

This is under review in freedomofpress/engineering#6.

[cfm]

I've broadened #6716 to reflect the steps that will be needed to apply this policy during the localization cycle for v2.6.0. The actual policy work scoped here is now done.