Maybe starting with the group of people who asked for this in the first place (Community Wishlist, Phab, ...?).

Starting by talking to people who voted for it in the wishlist survey is a good start, I think.

There's some discussion at T16950: [Epic] Support global preferences on Wikimedia wikis about this. Setting aside the grand goal of trying to implement every preference everywhere immediately, we're talking about having at least support for functionality such as a gender user preference that extends to more than one wiki (T134042). I continue to think that the obstacles to implementing such functionality are overblown and overstated. We have unified login, we have Meta-Wiki, we have centralized databases and tables available that each of the other 800-plus public Wikimedia wikis can query and write to, we normalized the user preferences schema years ago, etc. We should be in a place where we can implement at least basic support for global user preferences, in my opinion.

@MZMcBride: I agree. I don't see any technical barriers to global preferences at this point. The main issue is figuring out the needs of the various use cases and making sure that whatever UI we create meets those needs. During the unconference session, I did a demo of Lego's GlobalPreferences extension. The main concern brought up was that it doesn't provide any way to override the global settings. Some folks in the session suggested that a better solution might be to provide a way to easily copy settings across all wikis, but not have them actually be global. For example, if someone speaks English and Spanish, they might want to set their language to English on all wikis as a default, but set it to Spanish on es.wiki.

Discussion here:
https://meta.wikimedia.org/wiki/Talk:Community_Tech/Global_preferences

There will be an item explaining the request for feedback in Tech News that'll be distributed in a few hours. Additionally, I'm making sure a longer post is posted on a number of Village Pumps (examples: English, Swedish, German).

Posted on French WP, WIKT, WS and WV.

Of course, when you ask people to go to another wiki and post, some will reply on the wiki where they're being asked. Some discussions:

https://de.wikipedia.org/wiki/Wikipedia_Diskussion:Kurier#Globale_Einstellungen_.E2.80.93_sind_Ausnahmen_.28lokales_.C3.9Cberschreiben.29_gew.C3.BCnscht.3F

https://nl.wikipedia.org/wiki/Wikipedia:De_kroeg#Globale_voorkeuren_-_willen_jullie_lokale_.27overrides.27_.28aanpassing_aan_lokale_wiki.27s.29.3F

https://en.wiktionary.org/w/index.php?title=Wiktionary:Grease_pit/2017/August#Global_preferences_.E2.80.93_do_you_want_local_overrides.3F

In T154956#3561385, @Johan wrote:

Of course, when you ask people to go to another wiki and post, some will reply on the wiki where they're being asked. Some discussions:

https://de.wikipedia.org/wiki/Wikipedia_Diskussion:Kurier#Globale_Einstellungen_.E2.80.93_sind_Ausnahmen_.28lokales_.C3.9Cberschreiben.29_gew.C3.BCnscht.3F

https://nl.wikipedia.org/wiki/Wikipedia:De_kroeg#Globale_voorkeuren_-_willen_jullie_lokale_.27overrides.27_.28aanpassing_aan_lokale_wiki.27s.29.3F

https://en.wiktionary.org/w/index.php?title=Wiktionary:Grease_pit/2017/August#Global_preferences_.E2.80.93_do_you_want_local_overrides.3F

https://it.wikipedia.org/wiki/Wikipedia:Officina#Preferenze_globali_-_vogliamo_degli_override_locali.3F

Should we have the sentence "Please use the given link to provide feedback, otherwise we cannot ensure your feedback will definitely be seen" translated to most languages and be ready for copy&paste whenever that happens?

That's not a bad idea.

Also posted on the wikitech-ambassadors list if anyone there wants to spread the word.