First thing: in the core channel on Zulip every week (on Thursdays at 5:00pm Italian time) there is the meeting for core development (I always do my best to attend even if it is only to listen). Being that the i18n team needs help from the core team to make things smooth it is a good thing to start helping/listening there and give your two cents when you have something to say.
Also by being active in the comments section of the Issue I opened on GitHub and the PR(s) that could be opened in response to it you will be able to be that fly on the wall regarding this specific issue and also you will be i18n eyes to make sure that the specific change is made in a way it does not damage other things. I will be there too so that we can both keep our eyes open and ensure everything goes well.
Second, the i18n Team has a channel on Zulip too. As a team we should totally have a recurring monthly (or weekly) meeting set up like core has its weekly meeting happening always on same day at same time for the exact reason you mention, being able to discuss localization challenges and decisions to be made like the ones about themes to be localized.
On the topic of the default theme(s) localization: For now we localize the core - this takes strong precedence IMHO. Then in the same Crowdin space we can open the default theme(s) individual project(S) (so that when you open dashboard you will see a card for each project: ClassicPress, theme One, other default themes in the future) and by clicking the one you want to localize you can go in and translate strings. This setup has to be discussed however and if we are all in agreement we can proceed. I am not taking the decision on how to setup default themes localization alone, i18n is a Team and as a Team it should work.
On how Crowdin works I published a tutorial here, and generally I am the go-to person if you have doubts on how to use it or on i18n in ClassicPress.