Ha, there you are touching on a pet peeve of mine! Actually, several, even.
* from a legal point of view, I see no issue; and if there were, legal action would have been underway long since. The airport operating company being a private venture, they have every right to publish whatever they want in whichever language they prefer. (That makes a stark contrast to the Belgian railways, who respect the linguistical subtleties of this complicated country to great detail; much complicating some of their operations. For one example, in a train from Flanders to Brussels, the automatic announcements in synthesised speech will be in Dutch only, as long as it is on Flemish soil; the moment it enters bilingual territory, announcements switch to bilingual too. Same goes for the text announcements on the big LCD displays, where present).
* from the airport's commercial point of view, Chinese passengers deserve more linguistical care than German speakers, from whatever country: the latter are almost sure to understand the English information anyway; I am less certain about the Chinese, though I would expect those who can afford to travel to Europe to be upper-class, so reasonably well-educated; and, like as not, quite well prepared for cultural differences - much more than the stereotypical US'an visitor, for example.
* A scholar of languages, and of linguistical culture, I always regret the lack of cultural self-respect in the Germanic cultures. Scandinavians, for example, will often switch to English without even trying to open conversation in their own language - a shame! French speakers in Belgium have a bit too much of it, at times, even inventing neologist French-sounding names like "Craynhem"; the Dutch speakers learned to stand their ground over the last few decades, but Belgian Germanophones are very very docile - in my experience, most of them are perfectly quadrilingual, witness my several flights to EBBN (hehe, that's another one to look up!
) and they'll rarely insist on using their own language. But this gets us far from aviation, of course.
All in all, I see nothing surprising, even less anything illegal.