So the Chinese scenario is offensive because Chinese is spoken mostly only by actual Chinese people?

English is spoken mostly by caucasians. Therefore it should definitely be just as politically incorrect to make fun of the English language, by your logic.

I'd just like to know what exactly makes it racist to make fun of Chinese but OK to make fun of English, when neither thing being made fun of is a race.

white people tend to assume that they are the 'default', and thus don't think of it as a race

Well you'll be happy to know that I do not assume this, so it doesn't apply to me. If you try to use this again, it's a straw man.

THE ENGLISH LANGUAGE IS SEVERAL RACES

No it isn't. It's spoken by several races. Language does not equal race. They tend to go somewhat together, but they are distinct concepts. Making fun of a language is exactly that: it only makes fun of the language. You can't label it as racism when nobody is talking about race.

If the poster contained racial slurs or caricature drawings of Chinese people, yeah, I'd understand how that would be racist.

And you know, maybe there are people who know actual Chinese who are offended by this poster, just how there could be Italian speakers who are offended by Mario. But that still doesn't make it racism, since it does not deal with race.