I understand the desire to wait until after 1.0 to do GUI localization because of other priorities, but there is definitely a willingness from other developers to work on this now. I mean, multiplayer was a post 1.0 feature but there is the TES3MP project led by developers to address that right now. So long as there are guidelines about how we want to approach this, I don't see the downside of letting developers work on GUI localization now. Worst case, they could fork the project like TES3MP and merge it post 1.0, but it would of course be best to determine how it will look when we merge it back in.
I see that PR #1075 for adding flavor text to item descriptions was rejected because it would make the .omwaddon format incompatible with the .esm/.esp format, and a requirement for version 1.0 is that they should be compatible. Fair enough. While that would be a barrier if localizations were solely in .esp format, it sounds like the majority of commenters on this thread aren't liking that approach anyway and would prefer an external file, and this would avoid the issue with making .omwaddon incompatible.
If the only other reason is "because our core developers want to work on other stuff", then I don't see the downside of letting other developers add this functionality on their own. If there are other reasons for waiting until after 1.0, like the .omwaddon compatibility issue, then I'd be interested to know what they are.
For what it's worth, we have a few open PRs like #1290 Added hotkey for local/world map switching that are blocked because of a lack of a GUI localization strategy.