Mendacious

I think "habitual liar" would be OK in the context. It's something like: "X told us not to believe Y's story, because he is a habitual liar."

Maybe I should just say that, instead of trying to find an adjective.

Sounds good to me, although obviously I don't know the whole context. Maybe "describing him as a habitual liar" because "because" may suggest that you yourself are vouching for the truth of what follows, rather than still indirectly quoting X. Or "because (X said) he is a habitual liar."

And does anyone use the word at all?

There's a guy who writes China travel books and runs an email subscription list who likes the word. On the topic of China International Travel Service:

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Anyone who can remember dealing with CITS in the mid-80s, when most of China was still closed to foreign visitors, and the experience of travelling there was so different from today that it is very hard to imagine it, might care to differ. Anyone who has to put up with endless shopping stops, kick-back-driven hotel and restaurant choices, and entirely mendacious accounts of China's history and current affairs, might still be differing.
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On historical information available at the site of the Terracotta Warriors:

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Anyone expecting authenticity in these circumstances simply isn't paying attention to the inherently mendacious nature of the Chinese travel industry.
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A passage from “Thank You Mr Moto”, a thriller set in Beijing and written in 1964 by John P. Marquand which I bought on the recommendation that it contained some politically incorrect descriptions of the Chinese:

I recognized that he was lying deliberately with the hopeful mendacity of his race, which permits a Chinese to tell a blatant untruth even though he knows his falseness will be discovered a moment later.
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I can't remember hearing it "out loud" recently though it does have a pleasing sound to it, particularly in a plummy English accent such as my own.

I think I'll try and give it a whirl as soon as possible.

"untruthful," because that can refer to a single statement

Perhaps that's all the accusation is based on?

Perhaps that's all the accusation is based on?

Not in this case.

"because" may suggest that you yourself are vouching for the truth of what follows

That's true, and I wouldn't say it that way. This is what I have so far (with made-up names):

"Peter used his opening statement to convince the mediators that Sarah was a habitual liar and that they should not believe anything she said."

"the hopeful mendacity of his race"

Wow.

#11 -- Marquand died in 1960, so Thank You Mr Moto is unlikely to have been written in 1964. I think they were mostly from the thirties.

OK, wikipedia says 1936.

There was also a series of movies. Mr Moto, the clever and kindly Japanese detective, became Chinese c. 1941.

#15 -- That sounds fine.

OK, wikipedia says 1936.

So does the publishing information on the book itself, if I look properly.

Mr Moto, the clever and kindly Japanese detective, became Chinese c. 1941.

Did he indeed? Bad timing, what?

He wouldn't have been in China. I haven't seen any of the movies since I was maybe 10 years old, so I don't know if he was solving mysteries in the US at that point or maybe acting as a spy somewhere in the India-Burma theater or something. But Hollywood didn't abandon the franchise after Pearl Harbor, they just changed his nationality. The same thing happened to the Green Hornet's houseboy Kato. (on the radio)