To be clear, I'm not saying freedom of press between countries should be eliminated (quite the contrary!), only that diversity of languages should be preserved. In my opinion, this does two things:
1. Allows more diverse ideas to come into being, as language directly affects on one's way of thinking.
2. Slows the spread of dangerous ideas, due to some of their appeal (or the charisma of their advocates) being lost in translation.
For argument 1, that is known as the Sappir-Whorf hypothesis, is mostly discredited, at least in its stronger versions. By and large, people speaking the same language have a huge variety of differences of ideas, but you can also find areas with remarkable cultural similarity between people speaking different languages.
For an example of why this is not true in practice, look at Canada. The French speaking parts of Canada have far more in common with the rest of Canada and with the USA then they do with France, and even more so compared to Guinea or French Guiana.
The reason why language diversity should be preserved instead is that it gives access to a trove of cultural history that would be lost and forgotten, at least outside of scholarly circles, if it people could no longer understand it. This is by far the most problematic for oral cultural history, but even for written culture it is highly relevant.
For example, Westerners often label Japan as "xenophobic" (as did Biden recently), while the Japanese themselves do not see themselves as such. I theorize this is at least partly because the Japanese counterpart of "xenophobia" literally translates back as "foreigner hatred", which is a much stronger expression than "foreign-fear". This makes any argument using that expression sound ridiculous. "I'm not refusing to rent my house out to you because I hate you; I just don't know where you're coming from, what your lifestyle is like, and whether you pose a flight risk." This is within the gravity well of "xenophobia"; it is not within the gravity well of "外国人嫌悪". It is much harder to make the same argument when the language itself is working against you.
As to your other comment, a populist tells people what they want to hear. Any actual "cause" is only a means to an end for them. And Hitler was so good at being a populist that people wept at his speeches and became wholly subservient to his agenda. I think this was mostly due to his mastery of the language. It was the advertising slogans, not whatever he was selling. Had he had a larger audience to speak to, he would have changed the narrative accordingly.
One of the more infamous notions of purely cognitive S-W that dorm peddle that is entirely discredited is the idea that people who speak gendered languages tend to associate "feminine qualities" with feminine nouns describing even inanimate objects, and "masculine qualities" with the reverse. This has been measured in various ways and is simply false.
I would bet that your example of "foreigner hatred" vs "xenophobia" would also turn out to be wrong if studied. The much, much more likely reason Japanese people don't consider themselves xenophobic is that people don't typically think of themselves as holding bad views. I would bet lots of English-speaking white nationalists also don't consider themselves xenophobic for the same reason.
Finally, I think your outlook on the level to which Hitler manipulated the German people is highly optimistic about human nature. 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 wanted to do most of that and all they needed was someone who would allow and organize them. And to be clear, I'm not only speaking of the German people here - atrocities against Jewish people and other minorities were committed in many European countries where people didn't speak an ounce of German, they just needed to be let loose. Including, shamefully, my own country (Romania), to be clear that I'm not just pointing fingers at others.
