All opera conductors know Italian and German, possibly French, Russian, and Czech, although many libretti (such as Wagner's) aren't in the modern version of the language. When guest conducting away from your homeland, it's courteous if you make the effort to say something in the local language, even if it's only “good morning.” This was all the Russian conductor Evgeny Svetlanov could manage when he first came to London, so he employed a charming interpreter called Nina. She often needed to tone down his earthy vocabulary, but the color of her face as she translated usually showed the color of his language.

Even if you don't speak fluent Italian, rehearsing in Italy is made simpler by musical terms such as crescendo and sostenuto. Rehearsing in Germany is easier if you've studied Mahler's symphonies, because he used German rather than Italian for his instructions (lebhaft for allegro, schleppen for ritenuto, etc.).