Hi @al_shopov! Yes, as I said already when we spoke together, we already have the “pre-announcement of releases”. As shown in the link given by @schumaml, this is part of our checklist for each and every release! And it is the second item in our checklist! So it’s not even an option, or a “maybe” or “if we think about it”… It will be each and every time.
For GIMP 3.0.0, as noted in this report, we did open a thread already: GIMP's `master` branch string freeze
We updated this thread at every RC (e.g. for RC3). Not only this, every time we had a small update which may interfere with translations, we made a note about it in the thread.
These other threads you linked are other examples of such announcements (from the same release procedure), since we have been doing these since forever (actually since even before my time; back then my co-maintainer used to do the announcement threads on the GNOME internationalization mailing list; but all mailing lists are gone now and we were specifically asked to do these on Discourse instead; which we do!
).
It’s not hard, we do it every time, and we will continue to do it. 
As for accurate release dates, translators are the people who get the most accurate dates (additionally to developers), though these should never be considered as promises (e.g. the RC3 was not released last Saturday as I hoped; I clearly underestimated the ability to hack code while at FOSDEM!).
But anyway, if anything, actual release dates will either be on time or after “hoped” release dates (the ones noted in announcements to translators), not before.
The Discourse forum is, as far as we know, the only way left which was given to us to interact with translators (well apart from the Gitlab tracker itself, which you should feel very free to follow too). And in fact, we were specifically asked to make our announcements to translators there (I asked back when the mailing list was discontinued).
If this is not right, please see with other translator teams, then come back to us after and tell us how to make announcements instead. We are happy to comply to any reasonable process (and this current process is exactly what we did!) and we will just update our checklist. But please do this in agreement with other teams. We don’t want a different process per language team (we have 83 translations of GIMP so far!). 
That’s what we do with the i18n tag on Discourse (combined with the GIMP tag).
Our announcement for GIMP 3.0.0 release is even the fifth thread at the top of the i18n
tag at time of writing with 2200 views! So it looks like it has been quite “seen”! 
Now again: if you believe the procedure can be improved (of course it can; any procedure can be improved!), or a better/more accurate tag is needed, please discuss with other translator teams and come up with an accurate and unique procedure. Then just tell us. We’ll simply update our checklist, no questions asked (well… unless you propose something which seems really convoluted and time-wasting with doubtful reasons!
). All we want is to have translators happy and to have a simple procedure to follow, and repeat each and every time. We don’t want to “improvise” during a release.
Thanks! 