To me it doesn't make sense to say definitionally that you can't understand without expressing in an external language. First, my understanding even of English

To me it doesn't make sense to say definitionally that you can't understand without expressing in an external language. First, my understanding even of English is nuanced and not entirely shared. There are lots of times where I capture something in prose but feel and even explicitly state that the the words don't really completely capture the meaning and I'm relying on a shared understanding of the connotations of the words to convey what they do not. This implies that we have a strong grasp of our intended meaning that supercedes the meaning captured in language. Also, there have been many times where I'll learn a new word, especially words borrowed from other languages, and think, "Ah! Now I can more exactly express what I'm thinking!" My understanding hasn't changed at all, I'm just better able to express it in language. Another example: there are a lot of concepts, geo-spatial relationships between dimensional objects for example, that I never consciously verbalize, even internally, yet I can clearly hold in my head. It does seem plausible that this is another difference in mental models between people.