There are lots of ideas and discussions floating around out there, so we need to focus them and actually develop mockups for the first iteration of the potential UI changes for searching across projects using TextCat (language detection).
Trey sent an email with some v0 mockups that need improvement.
Chris suggested posting the mockups on a sub-page of the TextCat MediaWiki page for comment, which definitely sounds like a good idea.
If there isn't a consensus on the best UI, we could pursue A/B tests, and/or a demo for people to play with (T136637).
Below are a few options for the UI that can be considered; other mocks (for cross language display) are shown in T139310.
Additionally, here are a couple ways that other wiki's currently use to show their 'search results' across projects - enabling the user to easily search across projects for the same term(s):