A/N: Post LotTL AU. Ten/Simm!Master.
It's been beta-read by edzel2 – thank you!

They both spoke more languages than could be counted, and neither of them needed the Tardis to translate anything into their mother tongue. But even more than unnecessary, such a translation would be unwanted.

They switched between languages, some long dead and some non-existent yet for any given subjective point of time without any conscious effort. If one of them would use a dialect unknown to the other - hard task, finding it, but it had happened once or twice - he would quickly notice it and return to the safe one. The Tardis, if they used her help, translated everything into their native language, after all. And that language, the language of a race long since gone, the language only the two of them could still understand, the language practically dead, but still their own, sounded awkward to their ears.

They didn't talk in Gallifreyan. They probably didn't think in it.

It was a silent rule and quite an efficient shield.

The language of a telepathic race, it had always been far too precise, far too honest for their liking. It was virtually impossible to lie in it - oh, they could have said things which weren't true, but there was no such thing as ambiguity in Gallifreyan. Everything was defined to the core and so a lie would be obvious the moment they started to think of a proper tense. Telling the truth, they didn't have to make up meanings as they would come naturally, but both Time Lords would rather open their minds wide with no defences than actually admit anything in their first language.

So when the Master said "I hate you" in English it may have been true, but it may have been just teasing or a half-hearted attempt to hurt the Doctor with no real importance (or maybe he just wanted to say "I'll kill you first chance I'll get, until you won't regenerate anymore" or "why did you leave me, Thete, you stupid idiot" or "I love you"), but if he used Gallifreyan instead it would be a straightforward declaration. And if the Doctor ever said "I forgive you" in Gallifreyan, he would have to decide if he was just used to saying these words to the Master and didn't know what else to do, or maybe if he meant "you have done bad things, but we can start again" or "please stop, Koschei" or "I love you". And if it was all three of the above, it was always easier to pretend it was none.

There was a possibility, of course, that they meant exactly what they said.

But if they did, they would have used Gallifreyan.

They didn't. Not ever.