Sapir-Whorf, in its original form, claims language influences your perception of reality. Its "stronger" form, linguistic determinism, claims language
determines your thoughts. I claim neither of those things. I claim that, since language, as a highly intellectual cognitive function,
obviously has some sort of effect on your thoughts (but not to the point of determining them), having more of them makes us, as a collective, capable of having ideas we might not have had otherwise, some of which might be effective vaccines against mind-viruses (or mind-viruses themselves, in which case it's lucky we're not a monoculture).
That language affects thinking isn't the extraordinary claim, its contrary is. It is hard to believe that your method and efficiency of solving a problem is invariant to the format in which you represent that problem, especially when that format can be so varied.
> I think it's unfortunately quite clear that people didn't need a lot of sophisticated convincing to do what they did in WWII
They needed a decade of sophisticated propaganda and indoctrination, and even a change in the language itself, with words like "Übermensch" and "Lebensraum" being added to the dictionary. It didn't happen overnight.
I also don't know where you're getting the claim from that the countries where the Holocaust took place "didn't speak an ounce of German", given that a large part of it took place on former Austria-Hungary territory where German was presumably the most prevalent second language. But even supposing they didn't, how would that disprove my point? I'm not saying language barriers make you immune to mind-viruses, I'm saying they can slow down their spread. The effects of this one were devastating, but they could have been much more devastating without any language barrier whatsoever. Because one country I can realistically believe not to have spoken an ounce of German is the UK. And they had no shortage of like-minded individuals either, but the local moustache man, Mosley, while said to be a good orator, was no Hitler. I think the public having the ability to understand what Hitler was saying would have been demoralizing at best and galvanizing at worst. It definitely wouldn't have helped Churchill's cabinet, which at one point already had half the mind to strike a deal with him.
I'm aware that this is just a form of security through obscurity, but practice shows that security through obscurity often works. Might not be the strongest argument for preserving languages, but in my mind, a language monoculture is just like any other monoculture, with all its drawbacks.