I would like to expand upon their answers a bit. Let's take the German word "schadenfreude," which if you were to translate it into English would be "pleasure derived from the misfortune of another." That would be about the shortest translation you could make for it, because English doesn't have a word specific to that definition. Perhaps the shortest you could get without straying too far from the meaning would be "sadistic pleasure," but that implies a degree of maliciousness to it and that the person getting the pleasure is causing the misfortune or pain.
So, a translator translating from German to English has a choice to make when they come across the word: They can either translate the word as best they can or they can leave it as is. That's where the controversy arises. If they translate the word then they usually will change the meaning of the text in some way, and if they leave it as is then they open the door to allow "schadenfreude" into the English lexicon (which is exactly what has happened with "schadenfreude"). So, you wind up with people who argue back and forth about what the correct way to do it is, i.e. a controversy.
To more directly answer your question, "to become controversial" means "to become the source of argument or debate."
I would imagine that if this was from an article or something like that, that it's from Britain, as they tend to get all huffy about polluting the Queen's English with your foreign tongue or whatever. The other major English speaking countries don't really care that much. US English has been corrupted by other languages for as long as it's been around, Canadian English has been corrupted by pretty much every language that has also creeped into US English but especially French, and I don't even want to know what language has corrupted Australian English to come up with words like "kookaburra" and "kangaroo" or to name a place something like "Humpybong" (I know it's mostly Aboriginal).