Language of Truth: There is no first-class language of truth (that is more or less impossible), but Eldraeic was custom-designed to make speaking logical fallacies – by a variety of logics – bad mathematical statements, woeful imprecision, etc., etc., as difficult as possible if not downright ungrammatical, especially when spoken in formal registers.
While this does, as intended, make life more difficult for amateur liars, the general feeling is that even when professional liars (diplomats, say; see Will Not Tell A Lie) are called for, they really ought to be above that sort of cheap fast-talking anyway.
Will Not Tell A Lie: Not strictly played straight with the Imperials, but mostly played straight. They are very averse to lying, for reasons of honor/integrity, for reasons of ethics/religion (remember the appropriate verse of Elmiríën’s Truths?), for legal reasons based off of those (“falsification of information” is a public crime, and fraud, false claims, etc., are much more wide-ranging), and because as such a self-evidently superior and strong crowd, it really ought to be beneath them. Thus, whenever they can possibly get away with it, they play it straight.
And on such occasions as it is required (“necessary lies”, permitted to certain difficult professions and certain difficult circumstances), they much prefer to mislead – or better, let you mislead yourself – obfuscate, omit, exaggerate, imply, circumlocute, equivocate and filibuster their way to victory by, ah, carefully positioning the truth rather than actually lie (and certainly won’t ever break their word to do it). Their reputation for Will Not Tell A Lie is very useful, after all, and worth preserving.