avoided adding this in the past as on a multi user system it allows
one profile user to annoys others by changing the interface language,
but it comes at the expense of easily changing the language for users
who choose the wrong one on first startup
Closing anki when the preferences dialog
is open will cause an exception since
self.mw.col in Preferences is not available
anymore.
Avoid the exception by checking self.mw.col
again on accept().
