HOWTO guides for Mailman Suite administration
GNU Mailman / mailman-suite-doc
Related to the REST protocol use for communication with the Mailman core. You probably should not use this; such issues usually are specific to mailmanclient.
Requires little knowledge of Mailman internals to work on. Related to, but different from, "easy".
Requires little time and effort if you have requisite knowledge. Related to but different from "beginner-friendly".
Internationalization, including words that should be translated but aren't (bug) and languages that aren't available at all (wishlist).
User questions with no changes required in the Code
The UI makes things hard to do, or doesn't provide an operation.