The article states that people are upset because this move violates the Constitution and, by strengthening ties with Russia, it endangers Ukraine's chances of joining the European Union.
This isn't quite true.
See, Ukrainian and Russian are essentially the same language. The two are mutually intelligible, in fact. They're rather like American and Australian English--some words and pronunciations are different or strange, but two people who speak the languages can communicate perfectly well.
Ukrainians, however, DO NOT LIKE RUSSIA. Oh, the Ukrainians in the eastern part (who share a border with the old Soviet Union) may be willing to show some friendliness to Moscow, but if you ask Ukrainians on the street if they'd like to be part of the old USSR again, 99 out of 100 give a firm, "Ni!" After all, who enjoys being occupied?
As a result of the occupation, Ukrainians don't much like to admit commonalities with Russia, even though the commonalities go back well before World War II. The language similarity is a huge sticking point. The first time I visited Ukraine, I noted the similarity between Ukrainian and Russian, and the Ukrainians sharing the conversation became so enormously offended, only my status as an ignorant American saved me from instant pariah status. Curious, I brought it up again (rather more delicately) with someone else, and received much the same response. The associations with Russia are so strong that Ukrainians disavow having the same language. Do not ever confuse Ukrainian and Russian in Ukraine!
You'll start a riot.