I think that a good aspect of personal development, which has relevance for international sales, is the learning of other languages.
I learnt French and Latin in school - ok, I know I will not be selling anything by speaking Latin, but it does give you a good basis for learning the European languages that derive from it.
Additionally - I learnt Japanese in Japan, and can speak it at a basic sort of level.
I've also studied Swedish, Spanish, Dutch and Arabic - but I'm not sure whether I can really speak any of these.
What languages do you speak, or have you studied?