Add lang codes support

[BingLingGroup]

lang codes
Google says its using BCP-47 as a standard. According to my test (agermanidis/autosub pull request #136), a more specific and compliant with a standard's lang codes get a better result, which means they are able to avoid Google's IP-local optimization.
Obviously currently using Google recommended lang codes strategy is not enough. (Sometimes it's hard to compare due to the non-standard usage by Google.) We need a library to compare lang codes and perhaps give a full lang codes reference to user.