Unlike the place name in the profile's data field, which should be in the language and reflect the time of the person profiled, categories are in specific language streams. The English language stream is best developed, but if you click on "find" at the top right coner and then "categories" you'll see actual or potential streams in almost two dozen languages. So I would expect to see Aachen in both the English and German language streams.
A second principle which distinguishes us mightily from some other internet based genealogy sites is that we try to be historically accurate in our place names. So the question really is, what did the people of Aachen, and its ruler, see themselves part of in the year 800? While we refer to Charlemagne as the first Holy Roman Emperor, that specific term was not used until the 13th century. Certainly, Aachen was in neither "Germany" nor "Deutschland". Frankish Empire, perhaps?
Sorting this out may seem like a lot of trouble, but there are some benefits -- categories try to group like profiles together for genealogical benefit. If a category was siimply for Aachen, Germany, you would have Charlemagne grouped together with the woman who runs the bakery down the Aachen street in 2017. By giving Aachen a category name specific to a particular time period, one is not only historically accurate, but will group together others who were present in that place in that time period.