When a language reaches 20%, you can make a request on the translation issue tracker.
Once the language is added, announce it on the translation mattermost channel and also consider doing a blog post about it (involving/thanking the translators).
Languages available on Transifex
We often receive requests for new languages on Transifex, but because of some confusion in the user interface, many requests are for languages that already exist. For example, Russian might exist as "Russian (ru)" and Transifex will allow to request a new language for "Russian (ru_RU)". Transifex will also allow to request regional translations.
New language requests are often declined for this reason. If you would like to translate CiviCRM into a new language, please post a request on the translation issue tracker.
Policies on regional translations
Translating CiviCRM is a lot of work. Not only must CiviCRM itself be translated, but there are also extensions, documentation, website content, etc. We strongly encourage to avoid creating new regional translations when possible.
That said, there are regional translations that have a sufficiently large mass of users which are ready to do the work and support it in the long term.
If you would like to translate CiviCRM into a new regional language, please post a request on the translation issue tracker. Please argue your case: what kind of regional differences are there? can the "main" translation be adapted to be more neutral?
Once we create a new regional translation, we can also synchronize the translation efforts of the main 'core' translation, meaning that one or both languages can inherit the translations from the other language, but only for untranslated strings. This makes it possible to reduce translation efforts, and only have to review strings.