2154: fixed "translated from..." display.

Fixed to_localized_display to return the language name in the current
locale, instead of the locale of the language.

Rewrote the translation block to use blocktrans instead of multiple
trans lines.
2154: remove the start subtitle warning message

The message was pretty messed up for various reasons:
  - The check for when we should show it was wrong
    (unisubs.startdialog.Dialog.FORK_VALUE doesn't seem to be used
    anywmore)
  - The warning that we should show it from scratch was incorrect, we
    don't need to start from scratch anymore.

  - The warning that there are better choices for the from languages
    crashed and prevented the warning from being shown.  We could fix
    this code, but I don't think the check is correct in any case.

Let's just remove this code.  If we need to add something back, then
it's easy to do.