Linguistic imperialism occurs when people of one country try to tell people of another country how to spell and pronounce their superficially common language.
This typically happens when the one country has imposed its language on the other country sometime in the past. I have never seen it in cases where the sharing of a common language just developed naturally (e.g., I have never seen a German criticizing the way Austrians talk - they share a common language but pronounce it differently).
On the other hand, I have personally witnessed linguistic imperialism by European speakers of all four major languages of the Americas, i.e., English, Spanish, Portuguese, and French.
Please understand I am not saying that all European speakers of those four languages are linguistic imperialists. I am only saying I have witnessed cases of it happening. It was never pretty.
This happened during my four years in Rome, while I lived in an international house. We had a Spanish priest who was quite rude to Mexicans for not talking Castilian. We had a Portuguese priest who had the same attitude toward Brazilians.
We did not have any French in the house, but later I met a French Canadian who told me horror stories about his visit in Paris where people refused to understand him because his accent was different from theirs.
Last but not least, we had an English priest visiting with us for about a month or two (as it turned out he was to be made bishop, but that's a different story).
Now, this guy was actually a true British gentleman. Unlike the others he was not rude about it toward the Americans. Instead, he used a lot of teasing about it. However, he became less subtle when dealing with me. I'm from Slovakia, so English is an "acquired taste" for me.
He absolutely could not stand the fact I was talking in American English (with a Slovak accent, no doubt). He felt, and made it quite clear, that I had no right to use American English. When he got to know me better (and after I fired back a couple of times), he then switched to his tease mode even with me. But he was merciless and used any occasion he could find to pick on my English. For example, after we invited him to the best Japanese restaurant in Rome and were walking by a nearby fountain, I told him the water in all of Roman fountains was potable. To that he replied, "That means you can carry it." Of course, I said, "No, that would be porrrtable.
Anyway, enough examples, I suppose. It's time for a little commentary/editorial.
The people of the "mother countries" often say it is their language. To that I reply, no, it is not! The moment they imposed their language on another country, on another culture, that language became the language of the people of that country. Naturally, when spoken by the people of the "mother country," it is still theirs. It would be silly for Americans to tell the British how they should talk. But when it goes the other way, it is not just silly. It is linguistic imperialism (and I don't just mean the British, it just so happened I experienced it on my own skin from one of them, but it is a valid observation for the people of any "mother country".)
Whenever any of the "daughter countries" decided to separate from the mom, they chose to be independent. Often they had to go through a war to gain their independence. And being independent includes language independence. Even if they keep the name of the language, it is now their own language to do with as they please. That includes pronunciation, spelling, even grammar.
Sadly, I see examples of linguistic imperialism even on E2. When someone writes that color is the wrong American spelling of colour, that is linguistic imperialism. For what it's worth, color is the correct American spelling, while colour is the correct British spelling.
I am not saying all this to incite a war. For all I know, the noders who write things like that may think they are being funny. But after they check their rep, they all discover that quite a few people don't find it funny at all.
The purpose of this write-up is to make them aware that people of the "daughter countries" often find it offensive. Again, I have witnessed it with speakers of four different languages. The negative reaction was common to all four.
My personal advice: If you are joking, it is not funny. If you are serious, it is outright arrogant. Either way, please don't do it.