Funny thing about snow -- most of our planet's language-using inhabitants are familiar with it. The concept (at least some related concept cluster) exists in all of our minds. It's a relatively trivial matter to find a phrasing that expresses the right concept, when the concept already exists.
But, what about concepts that aren't quite so universal. What about concepts that simply are not shared? Well, we don't even need to look at fictional aliens to find examples. The discovery of new concepts and the sharing of foreign concepts between languages and cultures are historical facts. All we need to do is look at some of the Arabic words that English borrows -- zenith, nadir, azimuth, algebra, algorithm, cipher -- words we use for mathematical concepts.
These are words we would not have borrowed if we hadn't also borrowed the concepts. These are words that require paragraphs of text and sometimes full chapters in introductory textbooks before a beginning student feels that they are sufficiently defined.
The language of the Time Lords must include features that do not exist in any native language on Earth. If nothing else, there must exist terms used in trans-temporal engineering, acausal physics, sub-quantum mathematics and goodness knows how many other as-yet-undiscovered branches of science. And those are merely nouns. Those are the easy pieces of the language.
Even harder are the verbs. English verbs have tense, aspect, and mode -- properties that relate an utterance to time and to reality. There is no way that the Gallifreyan concepts of time or reality could resemble our own. They walk in Eternity. They meet each other out of order. There must exist, for example, a way to discuss an event which definitely happened in one person's past but is only likely to occur in another person's future and which, if it fails to occur for the other, will prevent these two people from ever meeting and speaking again in their shared subjective futures. It must be as easy to shift from "in flux for you, but fixed for me" to "fixed for both" as it is to shift from the indefinite "I ate" to the perfect "I have eaten".
If Gallifreyan is different enough from English, then it could easily take Amy months to learn how to understand a single Gallifreyan written utterance. She might need to learn a worldview that contradicts most of what her native worldview espouses. Even with the full support of the translation matrix, a few seconds is just not enough.
Then again, why would the translation matrix support it? Time Lord technology is notoriously difficult for other species to use. It's intentionally difficult as a matter of security. Time Lords don't want their toys to fall into the wrong hands.
If Time Lord translation matrices never translate written High Gallifreyan, then only Time Lords (or, at least, only educated Gallifreyans) can read the users' manuals for all these fun toys. Knowledge of the language itself can then be used as one part of a password system. Both Gallifrey and the universe at large are made safer by restricting access to this particular language.
By the way, if I could write in High Gallifreyan, I could have explained all of that in two or three fairly short sentences: The lingo-metric component of multi-factor authentication is made inherent to the development of psychic nth-dimensional cognition engines. In addition, there are natively-inherent limits to induced hypercognition in organic, manifold-embedded minds which complicate the matricized translation of ana-kataic languages to non-kataic speakers.
tl;dr
By the way, if I had written in High Gallifreyan, you wouldn't be able to understand it. You don't even know your ana from your kata. That's how I know you won't steal my TARDIS.
