--multiple tonalities, and which one you use changes the meaning of a given word COMPLETELY. As a result, in tonal languages like Chinese, all that the tonal variation that says "I'm a girl!" in English does is confuse people.
Expand this small principle to every feature of language.
As a result, your voice trainer should ALWAYS be a member of the community you're going to be communicating in, and if you have a significantly different one, you'll need more than one voice trainer, each an--