It is of course necessary to have reasonable command of the relevant language to follow courses or take up a work placement abroad. In general, the starting point is that you must have at least a command of the language to B2 level to be able to follow the programme (secondary school language exams at pre-university level). Radboud in'to Languages offers options to test your language skills.
However, each university will set out its own demands for exchange students, so ensure you find out beforehand what these are: what levels are demanded and how is this tested?
Do you still need to acquire an appropriate command of the language or would you like to freshen up your knowledge of this language? Then draw up a plan of approach well in advance; at least six months before departure. For example, you can follow language courses at Radboud In'to Languages and/or follow a language course at the location of your choice (German universities for instance offer such courses prior to a semester and are assessed as being excellent). Unfortunately you will need to bear the costs for any language course. But experience has shown the investment is more than worth it and taking a language course is a very nice way of getting to know other international students.