That's due to much the same reason why everyone appears to be speaking English unless they're in a room with someone who doesn't understand the language they're using, like when Lynn is around people speaking future-Latin. Maddy, Jasper, and Tokyo Rose are all speaking future-Japanese, after all.
Course I doubt this will be...as quick as she expects..
http://tvtropes.org/pmwiki/pmwiki.php/Main/TranslationConvention