When two people of the same culture speak the same language, they always understand one another.?
That would be false. Take us for instance. We speak English but we don't always know what the other is saying.
No, not always. Miscommunication is a part of life. Someone might not realize that what you said was meant to be cute & teasing and instead takes it to heart & their feelings are hurt. And over text, or email it makes things much worse for a chance of misunderstanding.