Language detection from polyglossia parameters

[945fc41467]

Is your feature request related to a problem? Please describe.
I would like ltex to automatic detect languages, without magic comment. I didn’t test, but it is supposed to work with babel package. I would be happy to have the same feature with polyglossia package.

Describe the solution you'd like
Just use the parameter set in

\usepackage{polyglossia}
    \setdefaultlanguage{Chinese}

and also take account temporary change in the text body.

Describe alternatives you've considered
Currently I use magic comments. It works but it is less elegant.