For Asian nationals, German can be quite a tongue twister, for some UK or US residents the language may have been learnt at school. Eatern and northern Europeans grasp quickly, for others the English language corporate environment is a huge hiderence to learning.
Whatever your standard, there are institutions here that will carry you on to proficiency. Not only for residents but also thos coming to Düsseldorf for an intense period of study. Furthermore, other language tuition is on offer too.