Authoritative source for translators
We (professional translators) generally check accepted sources such as official government publications from the government involved. We may take versions in a language that is not the native or strongest language for that country with a grain of salt, but often we have to accept them even if they sound bad. Remember that a translation being "official" has nothing to do with it being accurate!  Many official English translations of Chinese-language speeches or documents are really poor English, but we use the awkward sounding English formulations because they are the official version.

There are usually commonly-used formulaic utterances in Chinese (for example) that are accepted as having a particular meaning in English for diplomatic purposes. Again, that would be determined by referring to the small circle of official translators at high levels, or reference to published bilingual editions of speeches that come from official sources. Just because Johnny No-name says the word should be translated "X" and not "Y" doesn't make his version official (and, as has been said, just because the official version says "Y" and not "X" doesn't make that correct.)

Things get more interesting when you are interpreting in a court situation. At that time, there tends to be a lot of he-said, she-said going on as both sides challenge the exact wording chosen by the interpreter or translator whose work is being dissected. Of course both sides simply want their point of view to prevail. The fact is that language is not simple, and there's generally no absolute correct answer for what is the "correct" way to say something in another language unless the original statement is unbelievably simple. Even when it is, if a witness (or a government official!) wants to be non-responsive, the interpreter is left looking silly as he accurately interprets the response. I was in a court case once in a local court where the witness was asked how long the leash on a dog was (it was a dog attack case) and the witness actually answered "The leash was red." That's not an accuracy issue per se, but often if the audience doesn't understand the second language, it seems like it is a problem with the interpreter's accuracy.