How do Bolivians know what they are saying?

The same way you know what you're saying when you speak English! You know what the sounds mean to you and your brain associates them with things, ideas, and actions. For example, if I say "Bonjour" to a French person, "Guten Tag" to a German, or "goddag" to a Swede, each has learned since childhood that that particular sound is a greeting - i.e. what we associate with Hello.