Expose ILanguageManager as a REST resource, so that Postorius can use it to learn what languages a system supports, and thus provide list administrators with options for a list's preferred language. Refer to src/mailman/model/docs/languages.rst -- I imagine this will probably use a getUtility call to say,
getUtility(ILanguageManager)
Background, based on my conversation with Barry yesterday (I may have gotten some things wrong):
The system knows it supports certain languages. This is set in a config file that only the site administrator can change -- a change in the config file requires a mailman restart in order to take effect. The Language Manager is per system.
The Language Manager is a utility, like the user manager (that is, essentially a singleton in the system; interfaces that have multiple implementations are not utilities). When the system boots up, it registers implementations of interfaces. These are ZopeInterfaces, as we use the Zope component architecture, ZCA.
Also affects Postorius.
Once this is implemented in the REST API, change Postorius forms.py so it calls the API to ask that question when a list administrator is setting up a new list or changing preferences for a list, instead of hardcoding English as the only choice for the list's preferred language.