<html> has lang and xml:lang set via site language.
If user has different language set in preferences or uses ?uselang= override, then all interface messages should be wrapped in tag with appropriate lang & xml:lang attributes.
Version: unspecified
Severity: normal
This isn't generally feasible at the moment as there's no clear demarcation of what in output is a text boundary.
With the newer Message class infrastructure I think we should be able to more easily do this sort of thing.
When callers ask for an HTML or HTML-escaped version of a message, it could be wrapped (even if we have to explicitly ask for it at times).
I think the original problem identified in this ticket has been solved since. Bug 30238 is still a problem, but I don't think we should keep this open just for that. Perhaps expand that ticket to also include lang ?