"Declining standards" probably means English that (in the opinion of the author) has more and more incorrect but possibly understandable usages. To take a guess, saying things like "irregardless" or "come with John and I". Language change: the evolution of English over time, where new words and constructions become standard. For example, "internet", or the use of "impact" to mean "effect". "Geolinguistic diversity": usages that are standard in one variety of English but not, let's say, in BBC English. For example, the Scottish "where do you stay?" (meaning "live), or the Americanism "parking lot".

"Declining standards" refers to the way people judge something (their standards) not being as "high". That is, lower standards=lower expectations about the quality of something. For example: "Fast food has led to lower health standards." meaning that the ubiquity of convenient (but not healthy) food has made many lose their ability or desire to distinguish between good food and bad food. However, I don't fully understand the use do the term in the passage you gave. "Language change" and "geolinguistic diversity" make sense, but I don't see how the rise of English is connected to "declining standards". The passage doesn't even make clear what these standards are used to judge...

Thanks a lot.