I am a foreign student and I did not know German. But luckily, that was not a big problem for me, since the language center offers very good language courses. The courses are spread throughout the week at very comfortable times, such that we do not have to miss our regular lectures to attend these lectures.
In addition to this, the teachers are very proficient and often provide us with extra material (other than the coursebook) for more practice. They are also available after the lecture hours for doubt clearing.
Sometimes, the courses are also offered for free to the students who have German requirements for their degree programs.
In conclusion, I would definitely recommend taking language courses from the language center. It’s definitely better and a lot cheaper than other private language offering centers in Aachen.