Answer:
"Dance is the hidden language of the soul" means that dance connects to whoever is watching it and communicates so many beautiful feelings, that's why it is the "language of the soul". When you dance (or observe others dancing) it's like a whole different level of feeling. Dance is universal and anyone (dancer or not) can appreciate it no matter who you are or where you're from. Dancing is communicating feelings and emotions that are harder to express/convey with words.
“I speak two languages: English and body.” –Mae West