Would it not make sense for your employees or trainees to brush up on their English in a relaxed atmosphere?
You have the choice between effective one-on-one teaching or teaching in small groups - in any case, it will be at your premises in familiar surroundings and an informal atmosphere among colleagues.
During detailed preliminary talks, you and the potential participants of the language courses determine the lessons content to a large extent as only you and your employees know what matters in your company.