I have an answer from upstream: "They are independent. The one in the main dialogue is the default setting, which you can override per-language-group in the Detailed dialogue."
This is beginning to make sense to me, as you might have to increase the font size for an Arab locale (I'm just making something up...) -- Minimum font size settable in two places https://launchpad.net/malone/bugs/36808 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
