That’s a tough question! Thoughts we are aware of are part of consciousness. Consciousness is a topic of research that has been investigated more recently (for a while people thought it was a bit of a dodgy, not scientific concept). It seems we become conscious of something when a lot of the brain is interested in it, so a lot of different parts of the brain are processing this information, and the different parts are exchanging information with each other on this topic. The thing to remember is that we don’t really need to translate in a way our thoughts into something we understand, because they emerge from our own brain so it’s in a way the language of our brain and we grow up with it, so it’s clear to us. You can think of different types of thoughts too, some of them are dependent on language, like the inner voice we have in our head, and depends on the child having mastered speaking, but other thoughts can be visual, so remembering a painting, or yet you could remember an emotion, a feeling.