Here be dragons (and salt)
Sometimes the ties between the English language and Latin are interesting. Anglophones often pronounce Latin in an unexpected way, at least for the speakers of south-European languages like Spanish and Italian, who think they are closer to Latin anyway and therefore pronounce it right as a matter of fact. Many seem to believe so anyway (does anybody know how Latin was really pronounced? Theories abound). Perhaps the southerners are right, at least they do not diphthong vowels, which the Romans did not do either. Translators usually do not have a problem with this, as they have the written text to work on. Interpreters, howerver, can suffer. First they have to understand what the speaker is actually saying, then they have to decide if they want to repeat the Latin with a southern accent, so their listeners have a chance to grasp the Latin as it is, or if they would rather translate the Latin into Spanish (or Italian, in our example). So when an English speaker utters cogito ergo sum, which I have earnestly heard pronounced in non-intuitive and, at least for southerners, unexpected ways, the interpreter has to decide if he wants to say pienso, luego existo or if he repeats the Latin pronounced in a more Spanish way. This is, of course, one of the easier examples. The interesting case arises when a standard phrase in modern English is of Latin origin, has long ago been translated literally into English, but not into other languages, so the translation into these languages has to be into Latin. I know two examples that might illustrate what I mean: here be dragons and with a pinch (or grain) of salt. The first should be translated into Spanish and into German as hic sunt dracones, the second as cum grano salis. So are those expressions untranslatable? It depends on how you define untranslatable, but one could plausibly argue so. There is no equivalent in German or Spanish, so they might count as untranslatable into both languages. But they can still be translated into Latin. The interpreter can only hope his listeners understand these expressions in that language. In those cases, my gut feeling is that cum grano salis is generally known to Spanish and German speakers, hic sunt dracones less so. But as con una pizca de sal or mit einer Prise Salz and aquí hay dragones or hier gibt es Drachen don’t make any sense at all, the translation will have to be into Latin in both cases. What other possibility could there be?
By the way, the original expression cum grano salis seems to be modern Latin translated backwards according to contemporary grammatical rules. The original phrase is attributed to Pliny the Elder who in his Naturalis Historia, XXIII, 149, regarding an antidote to a poison, actually wrote addito salis grano.