Avoid crash in language_info

language_info calls:

  show_language_command (NULL, 1, NULL, NULL);

... "knowing" that show_language_command does not use its ui_file
parameter.  However, this was changed in commit 7514a661
("Consistently Use ui_file parameter to show callbacks").

This patch changes language_info to pass a ui_file.

It took a while to write the test -- this function is only called when
'verbose' is on and when switching the "expected" language in auto
mode.