Same language is a result of cultural similarity much more so than a cause for it. If there is no political or cultural pressure to speak the same language, they relatively quickly diverge.
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.
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.
