A blog user mentions the default search engine changes based on the selected locale (or the system locale if it is not en-US).
This is coming from here (but I'm not sure why it's the default search engine, instead of Google, in this case).
added component::applications/tor browser owner::tbb-team priority::medium severity::normal status::new tbb-mobile type::defect labels
Replying to sysrqb:
Specifically, DDG is not an option in this case.
mentioned in issue #31010 (moved)
moved to tpo/applications/tor-browser#30606 (closed)