[plt-dev] language dialog

Matthew suggested a change to the language dialog that I've just now
checked in. Please give it a try and let me know what you think.

Things known to be wrong:

  - no string-constants (so new text is only in English)

  - clicking the upper radio box does not make the selection
    go away from the other language choices (but that selection
    is ignored by the code when the upper radio box is
    chosen).

Other bugs you spot or other comments welcome.

Robby