When I travel I speak English. When I teach I speak Java, and for the same reason: it lets me be understood.
When I need to teach cross-platform subjects like DOM or data structures to mixed language audiences, I can easily use Java to show examples and everyone can follow them, even if they’re not Java programmers. Everyone reads at least pidgin Java. Only C++ programmers can read C++. Only Ruby programmers can read Ruby. (Still better than Perl though. No one can read Perl, Perl programmers included.)
Of course, if you’re traveling in one area, it’s better to speak the native language if you can. French gets you farther in Lyons than English, but if you’re at an international conference English is your only hope. If you’re at RubyConf, Ruby will get you far. However if you’re at OOPSLA or Software Development, Java is the way to go.
Java is the programmer’s lingua franca.
P.S. The Call for Papers for Software Development Best Practices 2007 has just been posted. Aside from a token C++ track, it’s a pretty language neutral conference; and we don’t normally bother to check presenters’ preferred programming language when deciding which presentations to accept. However, if you are showing code in a talk, I do recommend that you show it in Java.