They do so by turning "he has a point" into "who is this funny little moustache guy and why does he sound so angry?"
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.
However, in my opinion, the truth is that Hitler's ideas and the way they fit German culture at the time were far more powerful than his speech-giving skills. His speeches and style did not resonate with American or British or French audiences (to the extent that they didn't - he was in fact a pretty popular politician all over Europe before he began attacking them) not because they didn't understand German, but because they weren't Germans.
His speeches preyed on German bitterness after losing World War I, they preyed on German poverty because of the reparations, they preyed on German conceptions of national pride, and, of course, they preyed on German antisemitism and xenophobia more generally.
Many of those elements were not present in, say, France (who had defeated Germany in the recent war), so even if the French had spoken German, they certainly wouldn't have been swayed with arguments about how they had humiliated Germany, or about how Germany needs more "living space". The Spanish or the Polish would not have been swayed by Hitler's notion that blond blue-eyed Germans are the chosen people who rightfully rule the world. And the Japanese would have barely even understood what he was talking about, even if they had all been native German speakers.
And conversely, Hitler had no shortage of allies before and during WWII, not because Mussolini or Hirohito or Stalin were such fine speakers of German that they were mesmerized by his carefully crafted speeches, but because they liked his actual ideas and plans for conquest, regardless of language.
