I do have some questions, however. Firstly, how do we set the client to use Japanese? The updated launcher still only shows EN, DE, and FR as options.
Not for the other two already supported (DE and FR). These are already included in the English client. You can switch to them using a command in-game, for example to switch to German type /script SetCVar("Language.2", "de") in the chat. (Type /script SetCVar("Language.2", "en") to switch back.)