As someone who spends far too much time playing...
Around, say, 1 AM - 7 AM EST is a time when the least percentage of native-English speakers are on and Europeans are largely the majority. This is almost never an issue in any way. And I'm quite certain that this topic did not originate because it's a frequent issue from a majority of users, but a semi-unique case involving a few specific individuals. Having been around a lot, I know Zomise doesn't care when there's the occasional sporadic German/Spanish/French/whatever chat session with many people sharing their better-or-worse skills in the given language and being part of the community.
It was that, during these "off hours," a select few were essentially spamming chat in a single foreign language, even after other users had asked on numerous occasions that they form a clanchat or take it to PM. From what google translate could offer up, it seems a lot of these (in this case Swedish) general chat messages came down to were basically reporting what had happened. UserA is off doing something somewhere, UserB and UserC and UserD are all continuously asking where that certain somewhere is over the course of many messages...things which, even in English, would most likely be asked by the other users to be taken to /m or /clanchat.
At heart I don't think it's Zomise saying "100% English all the time", but pointing out that spamming general chat in a different language isn't often picked up on because it's 1) in another language so few people know what's being said (and how spammy it may be) and 2) the other language(s) stem from other timezones where fewer people might pick up on the spammy nature of the messages.
TL;DR - The point wasn't that English should be the only language spoken ever, but that a few individuals were spamming chat in a foreign language even after being asked to form a clanchat or take it to PM, the same kind of chat-spam that would be warned in English as well.