On iOS the WebState from which langauge is extracted represents the page/navigation (i.e., top level frame) while on the other platforms the WebContext is tied to whatever frame the form is in.

These should be consistent.

It's not immediately clear that either end of the frame tree is always better.

Perhaps the ideal would be to walk up the frame tree until we have a successfully detected language (i.e., if the form in in a frame that is in a different language than its parent).