• Would it be acceptable to render Makefile.PL interactive and ask which version of the documentation to install? Could it broke non-interactive installations by CPAN Testers?
• What about checking some locale variable to automatically make a choice? But my guess is it could be error prone, so i'm not sure.

I'd advice against both of those options, especially the last one, and that's because they make choice - or let the installer make a choice. It might be ok for a machine where there's just one user, and said user does the installation himself, but it's not a useful solution for a multiuser machine. What to do on box where some of the developers prefer to have the documentation in Japanese, others don't speak Japanese, and the installers happens to have his locale set to Japanese?

I'd say, install both. Now, you might wonder where. Well, that problem has already been solved. From the manual page of "man":

   LANG   If  LANG  is set, its value defines the name of the
          subdirectory where man first looks for  man  pages.
          Thus,  the  command  `LANG=dk man 1 foo' will cause
          man   to   look   for   the   foo   man   page   in
          .../dk/man1/foo.1,  and  if  it  cannot find such a
          file, then in .../man1/foo.1, where ... is a direc-
          tory on the search path.

So, if the 'normal' man pages go to say, /usr/local/man/man3, the Japanese pages would go to /usr/local/man/jp/man3. Unfortunally, I don't think MakeMaker has default support for this, nor will I guarantee that all man implementations support this.

Abigail