Change 305836 had a related patch set uploaded (by Danny B.):
Revert "Set Czech (cs) and Slovak (sk) as reciprocal fallbacks"

https://gerrit.wikimedia.org/r/305836

Plan

We need to manage the incident and too hasty merge, especially the lack of regular process followed (no technical news notice for example).

The DannyB's concerns, if shared by other users, are serious concerns.

So I offer the following plan:

1. Notify sk. and cs. Wikimedia communities, and mediawiki-l this Sunday
2. Gather feedback
3. Organize between Monday and Wednesday an office hour to discuss further the issue to take quickly a decision to keep in core or revert the change pending further discussion
4. If reverted, backport the revert to 1.28.0-wmf.16 branch at the next SWAT and deploy it on the Wikimedia cluster
5. Pursue discussion

Side note:

(no technical news notice for example)

I don't think we should rely on Tech News for something like this. Tech News is a good way to spread information that's relevant to a lot of different wikis, e.g. it affects all Wikimedia wikis or an entire project, or Wikidata or Commons which other wikis rely on, or concerns something new that might be interesting for other wikis to be aware of or try out as it's available for them or might be coming to them later.

If we're talking about fallback options for two specific languages, there are far more effective ways of reaching out to those particular communities, e.g. posting directly on the relevant Village Pumps, which will reach more local editors than a notice in Tech News will, at the same time as we're not e.g. taking up the time of our volunteer translators by having them translate it into Albanian and Portuguese.

Some point about the current situation.

Mailing list
8 days later, there is no reply related on the topic or objection on the mailing list.

Wikimedia projects
So far, @Danny_B seems to be the only contributor from a cs/sk Wikimedia wiki to have explicitly expressed the desire to revert the change, without any support.

We quickly addressed a namespace localisation issue in sk.wikipedia.

No notification has been sent to the Wikimedia projects (something I hoped Danny_B would be to support their request to revert the change), excepted by @LacoR on sk.wikipedia (where there isn't any reply).

TranslateWiki

On the other hand, on the TranslateWiki discussion, we can read a consensus to support the fallback (with, again, only @Danny_B opposition).

@Dvorapa says for example:

In my opinion the fallback to sk/cs is much better than default fallback to en (e.g. I´m the only one in my large family, who speaks English, but everybody understands Slovak).

Other software

According @Dvorapa, a fallback already existed in pywikibot.

As such, the Sunday 21 plan, beginning by if [these concerns about the fallback are] shared by other users didn't get further than step 2.

Change 305836 abandoned by Urbanecm:
[mediawiki/core@master] Revert "Set Czech (cs) and Slovak (sk) as reciprocal fallbacks"

Reason:

https://gerrit.wikimedia.org/r/305836