I suggested in the meetings to just override them (at least for major wikis) to make the change we want while preserving existing functionality
E.g. https://en.wikipedia.org/wiki/MediaWiki:Watchlist-details customizes it, but it might be okay to change the actual message text while preserving the watchlist notice. However, when I made that suggestion, I thought the old core text was closer to the override. Actually, the old core text was "{{PLURAL:$1|$1 page|$1 pages}} on your watchlist, not separately counting talk pages."
We should handle this here (or decide not to).
All overrides of the headers are listed at P5999.
New enwiki text (but the watchlist notice should be left alone):
""{{PLURAL:$1|'''$1''' page is|'''$1''' pages are}} on your [[Help:Watchlist|Watchlist]] (plus [[Help:Using talk pages|talk pages]])."