Hello Steve!
Welcome to the forum. I'm not working with the police so I can't tell you in which degree this language issue might be a problem.
I can however tell you about where I work, at Air Traffic Control on the Schiphol airport.
Even though (aviation) English is the main language for our job, Dutch is being spoken in all situations where English is not required or not enough to clear matters (f.i. with emergencies)
We have quite a lot foreign controllers working at ATC, from Denmark, Germany, South Africa and Spain. They have all been sent to "De Nonnetjes in Vught" which is a language institute (
click here) that works on teaching the Dutch language as fast as possible, as coherent as possible, as understandable as possible.
For the German guys it was easier to pick up on Dutch than for the South African couple but from -no Dutch at all- to -sufficient to converse with- in a matter of a couple weeks.
I can only imagine that, to work as a police officer it is recommended to speak Dutch fluently. For exact details I would advise to contact the appropriate contactpersons at the hiring department of the police.
I wish you good luck!