A big part of understand another language is learning that language's idioms. I would recommend looking up a list of the top most used English idioms. It helped me a lot with Spanish when I first started. Languages evolve, and idioms are a huge part of that evolution, unfortunately.

Hi diyaa! There is a huge variety of things that could be done, but you have done the first part, which is diagnosing and setting objectives. The main difference between Business English and General English is the vocabulary and texts you work with but, of course, if you are going to work in business, the best option will be to take a specific course. General English is also helpful but these contents can easily be integrated in a Business English course. I hope this helps you make your decision.