Well, killing a good percentage of your own people for their believes doesn't go down well with the rest of humanity. Plus, invading Russia in Winter without proper winter clothing makes one look like a complete idiot.
Things like these, plus, you know "being the enemy" and "starting a global conflict the likes of which the world has never seen before" won't make you win any popularity contest. And if most of the world doesn't like you, they are probably not going to embrace your culture and your language.
That's actually something that had a huge influence in the United States from the beginning of the first World War. There was a relatively large german speaking population in the U.S. in the early 1900's. WW1 and WW2 pretty much stamped that out. WW1 was also used as an argument for Prohibition (many of the large beer brewers were of german origin and held their business meetings in german).